diff --git a/.gitignore b/.gitignore
index 3429efd..631c483 100644
--- a/.gitignore
+++ b/.gitignore
@@ -114,3 +114,5 @@ MISC_AUDIT_INDEX.md
 HANDOVER_issue34_ble_esp32.md
 RELEASE_NOTES_v1.16.3.md
 PATH_HASH_AUDIT_INDEX.md
+MESHTIMESYNC_PLAN.md
+/meshtimesync-sim/
diff --git a/zephcore/ARCHITECTURE.md b/zephcore/ARCHITECTURE.md
index 0c21438..5cc329d 100644
--- a/zephcore/ARCHITECTURE.md
+++ b/zephcore/ARCHITECTURE.md
@@ -287,6 +287,44 @@ Direct (source-routed) packets bypass adaptive scaling entirely. They use minima
 - **Advertisements**: Ed25519 signature over (pubkey + timestamp + app_data)
 - **ACKs**: SHA-256(shared_secret + packet_hash) truncated to 4 bytes
 
+### 4.9 Mesh Time Sync (Clock Consensus)
+
+ZephCore-only divergence from Arduino MeshCore (like the Adaptive Contention Window). A node senses its own clock error from the Ed25519-signed timestamps in other nodes' adverts and — **opt-in, default off** (`set meshtimesync on`) — corrects it automatically. There is no trusted reference clock on a mesh, so this is a *consensus estimation* problem: the node assumes the majority of tenured advert senders within 3 flood hops is right. User-facing doc: `MESHTIMESYNC.md` at the repo root.
+
+**Module**: `helpers/MeshTimeSync.{h,cpp}` — role-agnostic estimator, owns no clock. Each role feeds it verified adverts (`onAdvertHeard`), calls `tick()` periodically (15-min pacing internal), and applies STEP verdicts under its own policy.
+
+**Sample table** (per-sender, `CONFIG_ZEPHCORE_TIMESYNC_TABLE_SIZE` slots: 32 default, 16 on RAM-bound companions; 24 B/slot):
+- 8-byte pubkey prefix — a security floor, not a tuning knob (shorter prefixes are grindable: an attacker could collide a tenured voter's prefix and reset its tenure with validly-signed adverts).
+- Latest advert timestamp (= the vote, per-sender monotonic — replays and flood dupes are inert) + arrival **uptime**. Skew is recomputed at evaluate time from the uptime anchor, so the node's own steps never stale stored samples.
+- Tenure tracking: first-heard uptime, advert count. Eligibility = heard ≥ 1 h, ≥ 2 adverts, latest sample ≤ 5 days old (bridges the 47 h flood-advert cadence).
+- Self-consistency: consecutive samples must satisfy `|Δadvert_ts − Δuptime| ≤ 45 s + 150 ppm × Δuptime`; violation (sender rebooted/corrected/lying) resets that sender's tenure.
+- **Hop-priority admission** (hop cap 3): a new sender may only displace a young entry farther (higher hop) than it; mature entries are protected unless silent > 24 h. Naive LRU churned hub nodes to zero eligible voters in simulation.
+
+**Consensus**: Marzullo interval intersection over eligible votes, each `[skew − r, skew + r]` with `r = 150 s + 15 s × hop` (the 150 s base covers the real fleet's good-clock scatter, not just RF delay). No absolute outlier thresholds against the local clock — clustering does the rejection, so an epoch-reset clock still finds the true cluster. Stepping requires `CONFIG_ZEPHCORE_TIMESYNC_QUORUM` (default 6, floor 3, build-time security knob) eligible senders AND a strict majority inside the intersection; otherwise abstain.
+
+**Correction policy** (priority: GPS > manual set > mesh consensus):
+- GPS gate: boards with GPS available + enabled never step (sensing continues).
+- Manual set (`time`, `clock sync`, app time set) arms a **7-day suppression** of all stepping, bootstrap included, plus drift-envelope pedigree.
+- Step trigger 10 min, dead band 5 min, step capped **±1 h**, one step per **6 h**, logged loudly. Production contains coherent wrong-time islands (+28 h × 63 repeaters at analysis time); the cap bounds capture drag to 4 h/day.
+- **Drift-envelope gate**: with a trusted sync + continuous uptime since (pedigree, RAM-only), corrections beyond `elapsed × 300 ppm + 10 min` are physically impossible for a crystal — refused regardless of quorum.
+- **Bootstrap**: local time < firmware build epoch (`FIRMWARE_BUILD_EPOCH`, CMake-injected) is provably wrong → any 3 agreeing senders, step to the cluster's **low edge** (midpoint − 150 s; undershoot so later refinement is always forward = monotonicity-safe).
+
+**Per-role step policy** (policy lives in the role, not the estimator):
+| Role | Policy | Why |
+|---|---|---|
+| Repeater | bidirectional | clock not load-bearing: forwarding/dedup/remote-admin run on `millis()`/hashes; a backward step only mutes own adverts at peers for a window equal to the step |
+| Observer | bidirectional | clock only stamps observations — exactly what this fixes |
+| Room server | forward-only | post timestamps feed client `sync_since` ordering |
+| Companion | forward-only | own clock stamps outgoing DMs; peers hold per-sender replay high-water marks |
+
+**Step application** (repeater reference, `applyTimeSyncStep`): set clock, one `zephcore_rtc_save` per step (never per evaluation), shift neighbor `heard_timestamp`s and ACL `last_activity` by the delta (unsigned "seconds ago" math), reset the login/anon/discover rate limiters.
+
+All policy timers (6 h rate limit, 7-day suppression, tenure, sample age) anchor on **uptime, never wall clock** — otherwise the very steps they govern would distort them.
+
+**CLI**: `set meshtimesync {on|off}`, `get meshtimesync` → state + live dry-run (eligible count, votes for/against, skew/radius, would-be verdict) + per-sender evidence table (full table over local USB; remote admin replies are summary-truncated to fit the packet). Sensing always runs, so the dry-run works before enabling.
+
+**Accepted limits**: a coordinated same-offset majority around a node captures it (no consensus survives that — Bitcoin timejacking lesson; mitigations: default-off, manual override, caps); sub-quorum islands abstain forever (bootstrap still heals dead clocks with 3 senders).
+
 ---
 
 ## 5. Radio Subsystem
diff --git a/zephcore/CMakeLists.txt b/zephcore/CMakeLists.txt
index e140042..258d2cf 100644
--- a/zephcore/CMakeLists.txt
+++ b/zephcore/CMakeLists.txt
@@ -427,6 +427,12 @@ project(zephcore)
 string(TIMESTAMP ZEPHCORE_BUILD_DATE "%Y %b %d" UTC)
 add_definitions(-DFIRMWARE_BUILD_DATE="${ZEPHCORE_BUILD_DATE}")
 
+# Build-time UNIX epoch: mesh time sync's "provably dead clock" floor (a local
+# time below this is impossible for a running build). string(TIMESTAMP) honors
+# SOURCE_DATE_EPOCH for reproducible builds.
+string(TIMESTAMP ZEPHCORE_BUILD_EPOCH "%s" UTC)
+add_definitions(-DFIRMWARE_BUILD_EPOCH=${ZEPHCORE_BUILD_EPOCH}u)
+
 # Single source of truth for the firmware version string (vMAJOR.MINOR.PATCH-zephyr).
 # Injected globally like FIRMWARE_BUILD_DATE above, so every app TU sees the same
 # value; the per-app `#ifndef FIRMWARE_VERSION` fallbacks only apply to builds that
@@ -473,6 +479,7 @@ target_sources(app PRIVATE
   adapters/gps/ZephyrGPSManager.cpp
   adapters/sensors/ZephyrEnvSensors.cpp
   helpers/AdvertDataHelpers.cpp
+  helpers/MeshTimeSync.cpp
   helpers/oled_power.c
   helpers/fatal_reboot.c
 )
diff --git a/zephcore/Kconfig b/zephcore/Kconfig
index 0e1c181..d5d774b 100644
--- a/zephcore/Kconfig
+++ b/zephcore/Kconfig
@@ -201,6 +201,31 @@ config ZEPHCORE_MAX_UNSYNCED_POSTS
 
 endif # ZEPHCORE_ROLE_REPEATER || ZEPHCORE_ROLE_ROOM_SERVER
 
+config ZEPHCORE_TIMESYNC_QUORUM
+	int "Mesh time sync: consensus quorum"
+	default 6
+	range 3 32
+	help
+	  Minimum eligible advert senders required before a mesh time-sync
+	  consensus may step the clock (a strict majority of them must also
+	  agree). Each quorum unit costs a local attacker one more physical
+	  radio sustained for tenure-hours, so lowering this trades eclipse
+	  resistance for coverage on small meshes (e.g. 7 repeaters = exactly
+	  6 potential voters). Floor 3 matches the bootstrap quorum — below
+	  that the interval intersection degenerates. Deliberately a build-time
+	  knob, not a runtime pref: this is a security parameter.
+
+config ZEPHCORE_TIMESYNC_TABLE_SIZE
+	int "Mesh time sync: sample table slots"
+	default 16 if ZEPHCORE_ROLE_COMPANION
+	default 32
+	range 8 64
+	help
+	  Per-sender advert sample slots (24 bytes each) for the mesh
+	  time-sync consensus. 32 fits dense neighborhoods; companions
+	  default to 16 (RAM-bound role). Must be at least the quorum for
+	  normal-mode stepping to be reachable.
+
 endmenu # Device Role
 
 if ZEPHCORE_ROLE_COMPANION
diff --git a/zephcore/MESHTIMESYNC.md b/zephcore/MESHTIMESYNC.md
new file mode 100644
index 0000000..8b34b9d
--- /dev/null
+++ b/zephcore/MESHTIMESYNC.md
@@ -0,0 +1,84 @@
+# Mesh Time Sync
+
+## What it does
+
+Your node listens to the timestamps inside other nodes' signed advertisements
+and, if its own clock is clearly wrong, gently corrects it — no GPS and no
+phone needed. It works on every role (repeater, room server, observer,
+companion). Why care: a node with a wrong clock shows garbage "last heard"
+times, and after a reboot without a time sync its own adverts can be silently
+ignored by every node that already knows it. On the live mesh today, almost
+half of all repeaters are more than an hour off; this feature fixes that
+class of problem automatically.
+
+Default is **OFF**. Nothing changes unless you enable it.
+
+## How to switch it on
+
+```
+set meshtimesync on
+```
+
+over the USB serial CLI or remote admin (repeaters, room servers, observers).
+On companions, toggle the `meshtimesync` custom variable from the app, or use
+the same command on the USB text CLI.
+
+Check what it is doing with:
+
+```
+get meshtimesync
+```
+
+This shows a live view even while the feature is off — how many trustworthy
+senders it can see, how many agree, what correction it *would* make. It is
+worth watching that dry-run output for a day before (and after) enabling.
+
+## What it will NOT do
+
+- It never overrides GPS time. Nodes with GPS enabled only observe.
+- It never overrides a recent manual `time <epoch>` or `clock sync` — any
+  manual set protects the clock from automatic changes for 7 days.
+- It never steps more than 1 hour at a time, and at most one step per 6 hours.
+- It does nothing unless at least 6 trustworthy senders are visible and a
+  strict majority of them agree (exception: a provably dead clock after a
+  reboot needs only 3 agreeing senders to recover).
+- Room servers and companions never step backward (that would break message
+  ordering / get their messages dropped as replays); a backward verdict is
+  only reported in `get meshtimesync`.
+
+## Drawbacks and honest limitations
+
+- Small meshes (fewer than ~7 advert-active nodes in range) mostly won't
+  reach quorum, so normal correction stays inactive — dead-clock recovery
+  after a reboot still works (needs only 3).
+- If most nodes around you share the SAME wrong time (it happens — one
+  region of the live mesh has a 60+ node island that is 28 hours fast), your
+  node may step toward them. The damage is bounded to 4 hours per day and
+  every step is visible in the logs and in `get meshtimesync`.
+- A backward step mutes this node's own adverts at peers for a window equal
+  to the step size. It self-heals as their clocks pass the old mark, or
+  delete + re-add the contact in the app to recover instantly.
+- Worst case — a coordinated wrong majority around you: turn it off
+  (`set meshtimesync off`) and set the clock manually, which also arms the
+  7-day protection.
+
+## Recovery recipe
+
+If the clock ever ends up AHEAD of true time (`time` alone will refuse with
+"clock cannot go backwards"):
+
+```
+set meshtimesync off
+clkreboot
+(log back in)
+time <correct epoch>
+set meshtimesync on
+```
+
+Works over remote admin too. The off/on bracket makes it deterministic (no
+bootstrap race), and the manual set arms the 7-day protection anyway. Fresh
+adverts take tens of minutes to accumulate after a reboot, so an operator
+syncing right after login always wins.
+
+For the design details (consensus algorithm, security model, why the limits
+are what they are), see [ARCHITECTURE.md](ARCHITECTURE.md) section 4.9.
diff --git a/zephcore/Repeater_CLI_commands.md b/zephcore/Repeater_CLI_commands.md
index dc8d6c0..0671b88 100644
--- a/zephcore/Repeater_CLI_commands.md
+++ b/zephcore/Repeater_CLI_commands.md
@@ -30,8 +30,8 @@ All commands are sent over USB serial (CDC-ACM). Commands sent remotely over the
 | Command | Description |
 |---------|-------------|
 | `clock` | Display current UTC time |
-| `clock sync` | Sync clock from the sender's timestamp (only advances, cannot go backwards) |
-| `time <unix_timestamp>` | Set RTC to a specific Unix timestamp (cannot go backwards) |
+| `clock sync` | Sync clock from the sender's timestamp (only advances, cannot go backwards). Arms the 7-day mesh-time-sync suppression window. |
+| `time <unix_timestamp>` | Set RTC to a specific Unix timestamp (cannot go backwards). Arms the 7-day mesh-time-sync suppression window. |
 
 ---
 
@@ -206,6 +206,7 @@ All `set uplink.*` changes are saved immediately and only applied after reboot.
 | `get radio.rxgain` | RX gain boost: `0` or `1` |
 | `get rxduty` | RX duty cycle mode: `0` or `1` |
 | `get gps duty` | Now-effective GPS duty interval in seconds (`always on (0)` when continuous) |
+| `get meshtimesync` | Mesh time-sync state + live dry-run: on/off, eligible voter count, votes for/against, consensus skew and radius, would-be verdict (`ok`/`in-band`/`step±N`/`abstain (reason)`/`hold (reason)`), step counters, suppression countdown, and a per-sender evidence table (`prefix hops count skew E`, `E` = tenure-eligible). Sensing runs even while off, so this works as a dry-run before enabling. Over remote admin the reply is truncated to the packet size (summary always fits); the full table needs the USB CLI. |
 | `get dc.restarts` | Duty-cycle preamble false-positive re-arm counter (RxTimeout re-arms + parked-RX watchdog recoveries). High values mean the preamble detector is tripping on noise/interference without real packets arriving — inflates RX-on time and drains battery; packets are never lost to it. Reset by `clear stats`. |
 | `get adc.multiplier` | Battery voltage ADC calibration multiplier |
 | `get bootloader.ver` | Bootloader version string |
@@ -250,6 +251,7 @@ Changes are persisted immediately unless noted. Some require a reboot.
 | `set radio.rxgain <0\|1\|on\|off>` | | RX gain boost, applied live. Replies `Error: unsupported` on radios without RX boost (SX127x); the pref is still saved. |
 | `set rxduty <0\|1\|on\|off>` | | RX duty cycle mode *(reboot required)*. Window timing auto-sized per SF/BW/preamble from the SX126x datasheet constraints (boot log line `rxduty:` shows the result). Zero-loss guarantee assumes senders on preamble-32 firmware (current MeshCore at SF≤8); legacy preamble-16 senders are only caught ~50% worst-phase — keep off until the local mesh has converted. Presets with 16-symbol preambles (SF≥9) fall back to continuous RX automatically. |
 | `set adc.multiplier <mult>` | (0 = use board default) | Battery voltage ADC calibration multiplier |
+| `set meshtimesync <on\|off>` | default **off** | Mesh time sync: automatically correct this node's clock from the consensus of Ed25519-signed advert timestamps heard on the mesh. Steps at most ±1 h per step, one step per 6 h; abstains without a quorum (default 6) of tenured agreeing senders; never overrides GPS or a manual set less than 7 days old. See `MESHTIMESYNC.md`. |
 | `set prv.key <hex>` | 64-char hex (32-byte key) | Replace private key; derive new identity *(reboot to apply)* |
 
 ---
diff --git a/zephcore/adapters/datastore/ZephyrDataStore.cpp b/zephcore/adapters/datastore/ZephyrDataStore.cpp
index 1b5499b..c04bbb3 100644
--- a/zephcore/adapters/datastore/ZephyrDataStore.cpp
+++ b/zephcore/adapters/datastore/ZephyrDataStore.cpp
@@ -663,6 +663,14 @@ void ZephyrDataStore::loadPrefs(NodePrefs &prefs)
 			prefs.rx_duty_cycle = 0;
 		}
 	}
+
+	/* Offset 151: meshtimesync (ZephCore extension, default 0 = off) */
+	if (off < len) {
+		prefs.meshtimesync = buf[off++];
+		if (prefs.meshtimesync > 1) {
+			prefs.meshtimesync = 0;
+		}
+	}
 }
 
 void ZephyrDataStore::savePrefs(const NodePrefs &prefs)
@@ -737,7 +745,9 @@ void ZephyrDataStore::savePrefs(const NodePrefs &prefs)
 	buf[off++] = (prefs.auto_shutdown_mv >> 8) & 0xFF;
 	/* Offset 150: rx_duty_cycle (ZephCore extension) */
 	buf[off++] = prefs.rx_duty_cycle;
-	/* Total: 151 bytes */
+	/* Offset 151: meshtimesync (ZephCore extension) */
+	buf[off++] = prefs.meshtimesync;
+	/* Total: 152 bytes */
 
 	bool ok = atomicReplaceFile(PREFS_FILE, buf, off);
 	LOG_DBG("savePrefs: wrote %s, ok=%d (%d bytes), name='%.16s'",
diff --git a/zephcore/app/CompanionMesh.cpp b/zephcore/app/CompanionMesh.cpp
index c0a9b38..eef252b 100644
--- a/zephcore/app/CompanionMesh.cpp
+++ b/zephcore/app/CompanionMesh.cpp
@@ -342,6 +342,56 @@ void CompanionMesh::loop()
 	}
 }
 
+void CompanionMesh::onAdvertTimeSample(const mesh::Identity &id, uint32_t timestamp,
+				       uint8_t hops)
+{
+	/* Signature already verified by mesh::Mesh before this hook fires. */
+	_timesync.onAdvertHeard(id.pub_key, timestamp, hops,
+				(uint32_t)(k_uptime_get() / 1000));
+}
+
+void CompanionMesh::timeSyncTick()
+{
+	if (!prefs.meshtimesync) return;
+
+	uint32_t up = (uint32_t)(k_uptime_get() / 1000);
+	uint32_t now = getRTCClock()->getCurrentTime();
+	MeshTimeSync::Verdict v = _timesync.tick(now, up);
+	if (v.type != MeshTimeSync::VERDICT_STEP) return;
+
+	/* GPS gate: sense only while GPS owns the clock. */
+	if (gps_is_available() && gps_is_enabled()) {
+		LOG_INF("meshtimesync: step %+d s wanted, GPS gate active - not applied",
+			(int)v.delta);
+		return;
+	}
+	/* Forward-only role: our clock stamps outgoing DMs and peers hold
+	 * per-sender replay high-water marks — a backward step gets our
+	 * messages dropped as replays. Report, never apply. */
+	if (v.delta < 0) {
+		_timesync.noteBackwardSkipped();
+		LOG_WRN("meshtimesync: backward step %+d s wanted - skipped (companion is forward-only)",
+			(int)v.delta);
+		return;
+	}
+	applyTimeSyncStep(v, now, up);
+}
+
+void CompanionMesh::applyTimeSyncStep(const MeshTimeSync::Verdict &v, uint32_t now,
+				      uint32_t uptime_secs)
+{
+	uint32_t new_time = (uint32_t)((int64_t)now + v.delta);
+	getRTCClock()->setCurrentTime(new_time);
+	zephcore_rtc_save(new_time);
+	time_sync_report(TIME_SYNC_MESH);
+
+	_timesync.noteStepApplied(v.delta, new_time, uptime_secs, v.bootstrap);
+	LOG_WRN("meshtimesync: stepped clock %+d s (%s, votes %u/%u) -> %u",
+		(int)v.delta, v.bootstrap ? "bootstrap" : "consensus",
+		(unsigned)v.consensus.votes_for, (unsigned)v.consensus.votes_against,
+		(unsigned)new_time);
+}
+
 void CompanionMesh::onLoginSent(const ContactInfo &contact)
 {
 	memcpy(&_pending_login, contact.id.pub_key, 4);
@@ -2237,6 +2287,7 @@ bool CompanionMesh::handleProtocolFrame(const uint8_t *data, size_t len)
 				getRTCClock()->setCurrentTime(secs);
 				time_sync_report(TIME_SYNC_APP);
 				zephcore_rtc_save(secs);  /* persist to hardware RTC */
+				_timesync.noteManualSync((uint32_t)(k_uptime_get() / 1000));
 				sendPacketOk();
 			} else {
 				sendPacketError(ERR_ILLEGAL_ARG);
@@ -2750,10 +2801,23 @@ bool CompanionMesh::handleProtocolFrame(const uint8_t *data, size_t len)
 			int n = snprintf(dp, remaining, "gps_interval:%u", (unsigned)gps_interval);
 			if (n > 0 && (size_t)n < remaining) {
 				dp += n;
+				first = false;
 			}
 			// If snprintf would have truncated, dp stays put — writeFrame
 			// sends only what we successfully wrote.
 		}
+		{
+			size_t remaining = (size_t)(rsp_end - dp);
+			if (!first && remaining > 0) {
+				*dp++ = ',';
+				remaining--;
+			}
+			int n = snprintf(dp, remaining, "meshtimesync:%d",
+					 prefs.meshtimesync ? 1 : 0);
+			if (n > 0 && (size_t)n < remaining) {
+				dp += n;
+			}
+		}
 		// Note: Environment sensors are auto-detected, no settings needed
 
 		writeFrame(rsp, (size_t)(dp - (char *)rsp));
@@ -2792,6 +2856,10 @@ bool CompanionMesh::handleProtocolFrame(const uint8_t *data, size_t len)
 					} else {
 						sendPacketError(ERR_ILLEGAL_ARG);
 					}
+				} else if (strcmp(key, "meshtimesync") == 0) {
+					prefs.meshtimesync = (val[0] == '1') ? 1 : 0;
+					_store->savePrefs(prefs);
+					sendPacketOk();
 				} else {
 					sendPacketError(ERR_ILLEGAL_ARG);
 				}
diff --git a/zephcore/app/CompanionMesh.h b/zephcore/app/CompanionMesh.h
index 24d9b95..ae58c54 100644
--- a/zephcore/app/CompanionMesh.h
+++ b/zephcore/app/CompanionMesh.h
@@ -6,9 +6,11 @@
 #pragma once
 
 #include <helpers/BaseChatMesh.h>
+#include <helpers/MeshTimeSync.h>
 #include <helpers/TransportKeyStore.h>
 #include <ZephyrDataStore.h>
 #include <NodePrefs.h>
+#include <zephyr/kernel.h>
 
 /* BLE push notification codes */
 #define PUSH_CODE_ADVERT              0x80
@@ -231,6 +233,15 @@ public:
 	bool onChannelLoaded(uint8_t idx, const ChannelDetails &ch) override;
 	bool getChannelForSave(uint8_t idx, ChannelDetails &ch) override;
 
+	/* Mesh time sync (forward-only: our clock stamps outgoing DMs and peers
+	 * hold per-sender replay high-water marks, so a backward step gets our
+	 * messages dropped as replays until wall-clock catches up) */
+	MeshTimeSync *getMeshTimeSync() { return &_timesync; }
+	void noteGPSTimeSync() { _timesync.noteGPSSync((uint32_t)(k_uptime_get() / 1000)); }
+	/* Paced evaluation — called from the housekeeping event (loop() only runs
+	 * on packet-driven events). */
+	void timeSyncTick();
+
 	/* Prefs (includes node_lat/lon) */
 	NodePrefs prefs;
 
@@ -369,6 +380,13 @@ private:
 	void flushDirtyContacts();
 	void flushDirtyChannels();
 
+	/* Mesh time sync */
+	MeshTimeSync _timesync{FIRMWARE_BUILD_EPOCH};
+	void onAdvertTimeSample(const mesh::Identity &id, uint32_t timestamp,
+		uint8_t hops) override;
+	void applyTimeSyncStep(const MeshTimeSync::Verdict &v, uint32_t now,
+		uint32_t uptime_secs);
+
 	/* Protocol version negotiation */
 	uint8_t _app_target_ver;
 
diff --git a/zephcore/app/ObserverMesh.cpp b/zephcore/app/ObserverMesh.cpp
index 478621e..d809a0b 100644
--- a/zephcore/app/ObserverMesh.cpp
+++ b/zephcore/app/ObserverMesh.cpp
@@ -9,6 +9,8 @@
 #include <mesh/Utils.h>
 #include <mesh/LoRaConfig.h>
 #include <adapters/radio/LoRaRadioBase.h>
+#include <adapters/clock/ZephyrRTCDiscover.h>
+#include <adapters/gps/ZephyrGPSManager.h>
 #include <helpers/MeshcoreJson.h>
 
 #include <zephyr/logging/log.h>
@@ -217,9 +219,80 @@ DispatcherAction ObserverMesh::onRecvPacket(Packet *pkt)
 	 * The same flood packet heard from different repeaters is published
 	 * separately, each with its own SNR/RSSI (propagation data). */
 	enqueuePacket(pkt);
+	harvestTimeSample(pkt);
 	return ACTION_RELEASE;  /* never retransmit */
 }
 
+/* ========== Mesh time sync ========== */
+
+void ObserverMesh::harvestTimeSample(Packet *pkt)
+{
+	if (pkt->getPayloadType() != PAYLOAD_TYPE_ADVERT) return;
+	if (pkt->getPathHashCount() > MeshTimeSync::HOP_CAP) return;
+	if (pkt->payload_len < PUB_KEY_SIZE + 4 + SIGNATURE_SIZE) return;
+
+	int i = 0;
+	Identity id;
+	memcpy(id.pub_key, &pkt->payload[i], PUB_KEY_SIZE);
+	i += PUB_KEY_SIZE;
+	uint32_t timestamp;
+	memcpy(&timestamp, &pkt->payload[i], 4);
+	i += 4;
+	const uint8_t *signature = &pkt->payload[i];
+	i += SIGNATURE_SIZE;
+
+	size_t app_data_len = pkt->payload_len - (size_t)i;
+	if (app_data_len > MAX_ADVERT_DATA_SIZE) app_data_len = MAX_ADVERT_DATA_SIZE;
+
+	uint8_t message[PUB_KEY_SIZE + 4 + MAX_ADVERT_DATA_SIZE];
+	int msg_len = 0;
+	memcpy(&message[msg_len], id.pub_key, PUB_KEY_SIZE); msg_len += PUB_KEY_SIZE;
+	memcpy(&message[msg_len], &timestamp, 4); msg_len += 4;
+	memcpy(&message[msg_len], &pkt->payload[i], app_data_len); msg_len += app_data_len;
+
+	if (!id.verify(signature, message, msg_len)) return;
+
+	_timesync.onAdvertHeard(id.pub_key, timestamp, pkt->getPathHashCount(),
+				(uint32_t)(k_uptime_get() / 1000));
+}
+
+void ObserverMesh::timeSyncTick()
+{
+	if (!_prefs.meshtimesync || !_rtc) return;
+
+	uint32_t up = (uint32_t)(k_uptime_get() / 1000);
+	uint32_t now = _rtc->getCurrentTime();
+	MeshTimeSync::Verdict v = _timesync.tick(now, up);
+	if (v.type != MeshTimeSync::VERDICT_STEP) return;
+
+	/* GPS gate: sense only while GPS owns the clock. */
+	if (gps_is_available() && gps_is_enabled()) {
+		LOG_INF("meshtimesync: step %+d s wanted, GPS gate active - not applied",
+			(int)v.delta);
+		return;
+	}
+	applyTimeSyncStep(v, now, up);
+}
+
+void ObserverMesh::noteTrustedTimeSync()
+{
+	_timesync.noteManualSync((uint32_t)(k_uptime_get() / 1000));
+}
+
+void ObserverMesh::applyTimeSyncStep(const MeshTimeSync::Verdict &v, uint32_t now,
+				     uint32_t uptime_secs)
+{
+	uint32_t new_time = (uint32_t)((int64_t)now + v.delta);
+	_rtc->setCurrentTime(new_time);
+	zephcore_rtc_save(new_time);
+
+	_timesync.noteStepApplied(v.delta, new_time, uptime_secs, v.bootstrap);
+	LOG_WRN("meshtimesync: stepped clock %+d s (%s, votes %u/%u) -> %u",
+		(int)v.delta, v.bootstrap ? "bootstrap" : "consensus",
+		(unsigned)v.consensus.votes_for, (unsigned)v.consensus.votes_against,
+		(unsigned)new_time);
+}
+
 /* ========== Serial CLI ========== */
 
 #define CLI_REPLY_SIZE 256
@@ -289,6 +362,12 @@ bool ObserverMesh::handleCLI(const char *command, char *reply, int reply_size)
 			snprintf(reply, reply_size, "%u",
 				 (_creds) ? (unsigned)_creds->mqtt_port : 8883u);
 
+		} else if (strcmp(key, "meshtimesync") == 0) {
+			_timesync.formatStatus(reply, reply_size,
+					       _rtc ? _rtc->getCurrentTime() : 0,
+					       (uint32_t)(k_uptime_get() / 1000),
+					       _prefs.meshtimesync != 0);
+
 		} else if (strcmp(key, "mqtt.tls") == 0) {
 			snprintf(reply, reply_size, "%u",
 				 (_creds) ? (unsigned)_creds->mqtt_tls : 1u);
@@ -392,6 +471,19 @@ bool ObserverMesh::handleCLI(const char *command, char *reply, int reply_size)
 				snprintf(reply, reply_size, "ERR cr must be 5-8");
 			}
 
+		} else if ((val = find_val(rest, "meshtimesync")) != nullptr) {
+			if (strcmp(val, "on") == 0) {
+				_prefs.meshtimesync = 1;
+				_store->savePrefs(_prefs);
+				snprintf(reply, reply_size, "meshtimesync=on");
+			} else if (strcmp(val, "off") == 0) {
+				_prefs.meshtimesync = 0;
+				_store->savePrefs(_prefs);
+				snprintf(reply, reply_size, "meshtimesync=off");
+			} else {
+				snprintf(reply, reply_size, "ERR must be on or off");
+			}
+
 		} else if (!_creds) {
 			snprintf(reply, reply_size, "ERR creds not initialized");
 
diff --git a/zephcore/app/ObserverMesh.h b/zephcore/app/ObserverMesh.h
index eba463d..c311bee 100644
--- a/zephcore/app/ObserverMesh.h
+++ b/zephcore/app/ObserverMesh.h
@@ -14,6 +14,7 @@
 #include <mesh/Identity.h>
 #include <mesh/RNG.h>
 #include <mesh/RTC.h>
+#include <helpers/MeshTimeSync.h>
 #include <helpers/NodePrefs.h>
 #include "RepeaterDataStore.h"
 #include "observer_creds.h"
@@ -58,6 +59,13 @@ class ObserverMesh : public Dispatcher {
 	void buildStatusJson(const char *status, char *out, size_t out_size);
 	uint32_t _start_uptime_secs;
 
+	/* Mesh time sync — the observer bypasses mesh::Mesh, so it verifies
+	 * advert signatures itself before harvesting. */
+	MeshTimeSync _timesync{FIRMWARE_BUILD_EPOCH};
+	void harvestTimeSample(Packet *pkt);
+	void applyTimeSyncStep(const MeshTimeSync::Verdict &v, uint32_t now,
+			       uint32_t uptime_secs);
+
 protected:
 	/* Capture RSSI + raw bytes before packet is parsed */
 	void logRxRaw(float snr, float rssi, const uint8_t raw[], int len) override;
@@ -84,6 +92,13 @@ public:
 	void publishSelfAdvert();
 	void publishStatus(const char *status);
 
+	/* Mesh time sync: paced evaluation (bidirectional stepping — the clock
+	 * is only load-bearing for observation timestamps, which is exactly
+	 * what this fixes). Driven from the 300 s status timer. */
+	void timeSyncTick();
+	/* SNTP just set the clock (trusted): arm suppression + drift envelope. */
+	void noteTrustedTimeSync();
+
 	/* Accessors used by main_observer.cpp */
 	NodePrefs *getNodePrefs()               { return &_prefs; }
 	const LocalIdentity &getSelfId() const  { return _self_id; }
diff --git a/zephcore/app/RepeaterDataStore.cpp b/zephcore/app/RepeaterDataStore.cpp
index 692b1a8..3293df5 100644
--- a/zephcore/app/RepeaterDataStore.cpp
+++ b/zephcore/app/RepeaterDataStore.cpp
@@ -201,6 +201,8 @@ bool RepeaterDataStore::loadPrefs(NodePrefs& prefs) {
      * read leaves the constructor defaults flood_max_unscoped=64, flood_max_advert=8). */
     fs_read(&file, &prefs.flood_max_unscoped, sizeof(prefs.flood_max_unscoped));
     fs_read(&file, &prefs.flood_max_advert, sizeof(prefs.flood_max_advert));
+    /* Mesh time sync (absent in <297-byte files; no-op EOF read keeps default 0 = off) */
+    fs_read(&file, &prefs.meshtimesync, sizeof(prefs.meshtimesync));
 
     fs_close(&file);
 
@@ -231,6 +233,7 @@ bool RepeaterDataStore::loadPrefs(NodePrefs& prefs) {
     if (prefs.rx_duty_cycle > 1) prefs.rx_duty_cycle = 0;
     if (prefs.apc_enabled > 1) prefs.apc_enabled = 0;
     if (prefs.apc_margin < 6 || prefs.apc_margin > 30) prefs.apc_margin = 16;
+    if (prefs.meshtimesync > 1) prefs.meshtimesync = 0;
 
     /* One-time format upgrade: old files (< 294 bytes) never saved the ZephCore
      * extension fields, and stored path_hash_mode/loop_detect as zero padding.
@@ -328,6 +331,8 @@ bool RepeaterDataStore::savePrefs(const NodePrefs& prefs) {
     /* Flood hop-ceiling extensions (extend the format past 294 bytes) */
     fs_write(&file, &prefs.flood_max_unscoped, sizeof(prefs.flood_max_unscoped));
     fs_write(&file, &prefs.flood_max_advert, sizeof(prefs.flood_max_advert));
+    /* Mesh time sync on/off (offset 296) */
+    fs_write(&file, &prefs.meshtimesync, sizeof(prefs.meshtimesync));
 
     ret = fs_sync(&file);
     fs_close(&file);
diff --git a/zephcore/app/RepeaterMesh.cpp b/zephcore/app/RepeaterMesh.cpp
index 374bad9..db5da08 100644
--- a/zephcore/app/RepeaterMesh.cpp
+++ b/zephcore/app/RepeaterMesh.cpp
@@ -12,6 +12,8 @@
 #include <adapters/sensors/SimpleLPP.h>
 #include <adapters/sensors/ZephyrEnvSensors.h>
 #include <adapters/gps/ZephyrGPSManager.h>
+#include <adapters/clock/ZephyrRTCDiscover.h>
+#include <helpers/time_sync.h>
 #include <zephyr/kernel.h>
 #include <zephyr/device.h>
 #include <zephyr/devicetree.h>
@@ -711,6 +713,10 @@ void RepeaterMesh::onAdvertRecv(mesh::Packet* packet, const mesh::Identity& id,
                                 const uint8_t* app_data, size_t app_data_len) {
     mesh::Mesh::onAdvertRecv(packet, id, timestamp, app_data, app_data_len);
 
+    /* Signature already verified by mesh::Mesh before this hook fires. */
+    _timesync.onAdvertHeard(id.pub_key, timestamp, packet->getPathHashCount(),
+                            (uint32_t)(k_uptime_get() / 1000));
+
     if (packet->getPathHashCount() == 0 && !isShare(packet)) {
         AdvertDataParser parser(app_data, app_data_len);
         if (parser.isValid() && parser.getType() == ADV_TYPE_REPEATER) {
@@ -1374,11 +1380,71 @@ void RepeaterMesh::loop() {
     }
 #endif
 
+    timeSyncTick();
+
     uint32_t now = k_uptime_get();
     uptime_millis += now - last_millis;
     last_millis = now;
 }
 
+void RepeaterMesh::timeSyncTick() {
+    if (!_prefs.meshtimesync) return;
+
+    uint32_t up = (uint32_t)(k_uptime_get() / 1000);
+    uint32_t now = getRTCClock()->getCurrentTime();
+    MeshTimeSync::Verdict v = _timesync.tick(now, up);
+    if (v.type != MeshTimeSync::VERDICT_STEP) {
+        if (v.type == MeshTimeSync::VERDICT_ABSTAIN) {
+            LOG_DBG("meshtimesync: abstain (%s)", MeshTimeSync::reasonStr(v.reason));
+        }
+        return;
+    }
+    /* GPS gate: GPS sets the clock unconditionally — a wrong mesh step
+     * followed by a GPS step-back would poison our own advert high-water
+     * marks at peers. Sense only. */
+    if (gps_is_available() && gps_is_enabled()) {
+        LOG_INF("meshtimesync: step %+d s wanted, GPS gate active - not applied",
+                (int)v.delta);
+        return;
+    }
+    applyTimeSyncStep(v, now, up);
+}
+
+void RepeaterMesh::applyTimeSyncStep(const MeshTimeSync::Verdict& v, uint32_t now,
+                                     uint32_t uptime_secs) {
+    uint32_t new_time = (uint32_t)((int64_t)now + v.delta);
+    getRTCClock()->setCurrentTime(new_time);
+    zephcore_rtc_save(new_time);
+    time_sync_report(TIME_SYNC_MESH);
+
+    /* Wall-clock-anchored bookkeeping must move with the step, or a backward
+     * step underflows the unsigned "seconds ago" math. 0 = unset sentinel. */
+#if MAX_NEIGHBOURS > 0
+    for (int i = 0; i < MAX_NEIGHBOURS; i++) {
+        if (neighbours[i].heard_timestamp == 0) continue;
+        int64_t shifted = (int64_t)neighbours[i].heard_timestamp + v.delta;
+        neighbours[i].heard_timestamp = (shifted > 0) ? (uint32_t)shifted : 1;
+    }
+#endif
+    for (int i = 0; i < acl.getNumClients(); i++) {
+        ClientInfo* c = acl.getClientByIdx(i);
+        if (c->last_activity == 0) continue;
+        int64_t shifted = (int64_t)c->last_activity + v.delta;
+        c->last_activity = (shifted > 0) ? (uint32_t)shifted : 1;
+    }
+    /* A backward step would otherwise wedge these shut until wall-clock
+     * catch-up. */
+    discover_limiter.reset();
+    anon_limiter.reset();
+    login_fail_limiter.reset();
+
+    _timesync.noteStepApplied(v.delta, new_time, uptime_secs, v.bootstrap);
+    LOG_WRN("meshtimesync: stepped clock %+d s (%s, votes %u/%u) -> %u",
+            (int)v.delta, v.bootstrap ? "bootstrap" : "consensus",
+            (unsigned)v.consensus.votes_for, (unsigned)v.consensus.votes_against,
+            (unsigned)new_time);
+}
+
 bool RepeaterMesh::hasPendingWork() const {
     return _mgr->getOutboundTotal() > 0;
 }
diff --git a/zephcore/app/RepeaterMesh.h b/zephcore/app/RepeaterMesh.h
index 5628558..4f54106 100644
--- a/zephcore/app/RepeaterMesh.h
+++ b/zephcore/app/RepeaterMesh.h
@@ -18,6 +18,7 @@
 #include <mesh/SimpleMeshTables.h>
 #include <helpers/ClientACL.h>
 #include <helpers/CommonCLI.h>
+#include <helpers/MeshTimeSync.h>
 #include <helpers/RegionMap.h>
 #include <helpers/TransportKeyStore.h>
 #include <helpers/RateLimiter.h>
@@ -112,6 +113,7 @@ class RepeaterMesh : public mesh::Mesh, public CommonCLICallbacks {
     uint8_t pending_sf;
     uint8_t pending_cr;
     int matching_peer_indexes[MAX_CLIENTS];
+    MeshTimeSync _timesync{FIRMWARE_BUILD_EPOCH};
 #if IS_ENABLED(CONFIG_ZEPHCORE_REPEATER_UPLINK)
     ObserverCreds _uplink_creds;
     bool _uplink_reboot_required;
@@ -131,6 +133,8 @@ class RepeaterMesh : public mesh::Mesh, public CommonCLICallbacks {
 #endif
 
     void putNeighbour(const mesh::Identity& id, uint32_t timestamp, float snr);
+    void timeSyncTick();
+    void applyTimeSyncStep(const MeshTimeSync::Verdict& v, uint32_t now, uint32_t uptime_secs);
     uint8_t handleLoginReq(const mesh::Identity& sender, const uint8_t* secret, uint32_t sender_timestamp, const uint8_t* data, bool is_flood);
     uint8_t handleAnonRegionsReq(const mesh::Identity& sender, uint32_t sender_timestamp, const uint8_t* data, size_t data_len);
     uint8_t handleAnonOwnerReq(const mesh::Identity& sender, uint32_t sender_timestamp, const uint8_t* data, size_t data_len);
@@ -244,6 +248,10 @@ public:
     void saveIdentity(const mesh::LocalIdentity& new_id) override;
     void clearStats() override;
 
+    /* Mesh time sync */
+    MeshTimeSync* getMeshTimeSync() override { return &_timesync; }
+    void noteGPSTimeSync() { _timesync.noteGPSSync((uint32_t)(k_uptime_get() / 1000)); }
+
     /* Adaptive contention window callbacks */
     float getContentionEstimate() const override {
         return getContentionTracker().getContentionEstimate();
diff --git a/zephcore/app/RoomServerMesh.cpp b/zephcore/app/RoomServerMesh.cpp
index 9e97751..75f63a8 100644
--- a/zephcore/app/RoomServerMesh.cpp
+++ b/zephcore/app/RoomServerMesh.cpp
@@ -19,6 +19,8 @@
 #include <adapters/sensors/SimpleLPP.h>
 #include <adapters/sensors/ZephyrEnvSensors.h>
 #include <adapters/gps/ZephyrGPSManager.h>
+#include <adapters/clock/ZephyrRTCDiscover.h>
+#include <helpers/time_sync.h>
 #include <zephyr/kernel.h>
 #include <zephyr/device.h>
 #include <zephyr/devicetree.h>
@@ -1104,11 +1106,70 @@ void RoomServerMesh::loop() {
         dirty_contacts_expiry = 0;
     }
 
+    timeSyncTick();
+
     uint32_t now = k_uptime_get();
     uptime_millis += now - last_millis;
     last_millis = now;
 }
 
+void RoomServerMesh::onAdvertRecv(mesh::Packet* packet, const mesh::Identity& id,
+                                  uint32_t timestamp, const uint8_t* app_data,
+                                  size_t app_data_len) {
+    (void)app_data; (void)app_data_len;
+    /* Signature already verified by mesh::Mesh before this hook fires. */
+    _timesync.onAdvertHeard(id.pub_key, timestamp, packet->getPathHashCount(),
+                            (uint32_t)(k_uptime_get() / 1000));
+}
+
+void RoomServerMesh::timeSyncTick() {
+    if (!_prefs.meshtimesync) return;
+
+    uint32_t up = (uint32_t)(k_uptime_get() / 1000);
+    uint32_t now = getRTCClock()->getCurrentTime();
+    MeshTimeSync::Verdict v = _timesync.tick(now, up);
+    if (v.type != MeshTimeSync::VERDICT_STEP) return;
+
+    /* GPS gate: sense only while GPS owns the clock. */
+    if (gps_is_available() && gps_is_enabled()) {
+        LOG_INF("meshtimesync: step %+d s wanted, GPS gate active - not applied",
+                (int)v.delta);
+        return;
+    }
+    /* Forward-only role: post timestamps feed client sync_since ordering,
+     * so a backward step corrupts message sync. Report, never apply. */
+    if (v.delta < 0) {
+        _timesync.noteBackwardSkipped();
+        LOG_WRN("meshtimesync: backward step %+d s wanted - skipped (room server is forward-only)",
+                (int)v.delta);
+        return;
+    }
+    applyTimeSyncStep(v, now, up);
+}
+
+void RoomServerMesh::applyTimeSyncStep(const MeshTimeSync::Verdict& v, uint32_t now,
+                                       uint32_t uptime_secs) {
+    uint32_t new_time = (uint32_t)((int64_t)now + v.delta);
+    getRTCClock()->setCurrentTime(new_time);
+    zephcore_rtc_save(new_time);
+    time_sync_report(TIME_SYNC_MESH);
+
+    /* Wall-clock-anchored bookkeeping moves with the step (see RepeaterMesh). */
+    for (int i = 0; i < acl.getNumClients(); i++) {
+        ClientInfo* c = acl.getClientByIdx(i);
+        if (c->last_activity == 0) continue;
+        int64_t shifted = (int64_t)c->last_activity + v.delta;
+        c->last_activity = (shifted > 0) ? (uint32_t)shifted : 1;
+    }
+    login_fail_limiter.reset();
+
+    _timesync.noteStepApplied(v.delta, new_time, uptime_secs, v.bootstrap);
+    LOG_WRN("meshtimesync: stepped clock %+d s (%s, votes %u/%u) -> %u",
+            (int)v.delta, v.bootstrap ? "bootstrap" : "consensus",
+            (unsigned)v.consensus.votes_for, (unsigned)v.consensus.votes_against,
+            (unsigned)new_time);
+}
+
 bool RoomServerMesh::hasPendingWork() const {
     return _mgr->getOutboundTotal() > 0;
 }
diff --git a/zephcore/app/RoomServerMesh.h b/zephcore/app/RoomServerMesh.h
index b3d67fd..d7354a0 100644
--- a/zephcore/app/RoomServerMesh.h
+++ b/zephcore/app/RoomServerMesh.h
@@ -19,6 +19,7 @@
 #include <mesh/SimpleMeshTables.h>
 #include <helpers/ClientACL.h>
 #include <helpers/CommonCLI.h>
+#include <helpers/MeshTimeSync.h>
 #include <helpers/RegionMap.h>
 #include <helpers/TransportKeyStore.h>
 #include <helpers/RateLimiter.h>
@@ -94,9 +95,12 @@ class RoomServerMesh : public mesh::Mesh, public CommonCLICallbacks {
     uint8_t pending_sf;
     uint8_t pending_cr;
     int matching_peer_indexes[MAX_CLIENTS];
+    MeshTimeSync _timesync{FIRMWARE_BUILD_EPOCH};
 
     int handleRequest(ClientInfo* sender, uint32_t sender_timestamp, uint8_t* payload, size_t payload_len);
     mesh::Packet* createSelfAdvert();
+    void timeSyncTick();
+    void applyTimeSyncStep(const MeshTimeSync::Verdict& v, uint32_t now, uint32_t uptime_secs);
 
     /* Room server: shared-post buffer + push-to-client sync */
     void addPost(ClientInfo* client, const char* postData);
@@ -145,6 +149,7 @@ protected:
 
     bool filterRecvFloodPacket(mesh::Packet* pkt) override;
 
+    void onAdvertRecv(mesh::Packet* packet, const mesh::Identity& id, uint32_t timestamp, const uint8_t* app_data, size_t app_data_len) override;
     void onAnonDataRecv(mesh::Packet* packet, const uint8_t* secret, const mesh::Identity& sender, uint8_t* data, size_t len) override;
     int searchPeersByHash(const uint8_t* hash) override;
     void getPeerSharedSecret(uint8_t* dest_secret, int peer_idx) override;
@@ -192,6 +197,11 @@ public:
     void saveIdentity(const mesh::LocalIdentity& new_id) override;
     void clearStats() override;
 
+    /* Mesh time sync (forward-only: post timestamps feed client sync_since
+     * ordering, so a backward step would corrupt message sync) */
+    MeshTimeSync* getMeshTimeSync() override { return &_timesync; }
+    void noteGPSTimeSync() { _timesync.noteGPSSync((uint32_t)(k_uptime_get() / 1000)); }
+
     /* Adaptive contention window callbacks */
     float getContentionEstimate() const override {
         return getContentionTracker().getContentionEstimate();
diff --git a/zephcore/app/main_observer.cpp b/zephcore/app/main_observer.cpp
index c51d096..6c7a144 100644
--- a/zephcore/app/main_observer.cpp
+++ b/zephcore/app/main_observer.cpp
@@ -173,11 +173,13 @@ static void print_banner(void)
 	cli_println("set sf        <7-12>       Spreading factor");
 	cli_println("set bw        <idx>        Bandwidth: 3=62.5 0=125 1=250 2=500 kHz");
 	cli_println("set cr        <5-8>        Coding rate");
+	cli_println("set meshtimesync <on|off>  Mesh clock consensus correction");
 	cli_println("");
 	cli_println("--- Query ---");
 	cli_println("get wifi.status            WiFi connection state");
 	cli_println("get mqtt.status            MQTT connection state");
 	cli_println("get radio                  LoRa radio parameters");
+	cli_println("get meshtimesync           Time-sync consensus state (dry-run)");
 	cli_println("help                       Show this screen");
 	cli_println("=========================");
 }
@@ -228,9 +230,14 @@ static void process_cli_rx(void)
 
 static mesh::ZephyrRTCClock s_rtc_clock;
 
+/* SNTP callback runs on the WiFi thread; the mesh time-sync module is
+ * main-thread-only, so just flag the sync and let the event loop arm it. */
+static atomic_t s_sntp_synced;
+
 static void time_sync_cb(uint32_t unix_ts)
 {
 	s_rtc_clock.setCurrentTime(unix_ts);
+	atomic_set(&s_sntp_synced, 1);
 	LOG_INF("RTC synced from SNTP: %u", unix_ts);
 }
 
@@ -366,8 +373,13 @@ int main(void)
 		if (ev & MESH_EVENT_CLI_RX) {
 			process_cli_rx();
 		}
+		if (atomic_cas(&s_sntp_synced, 1, 0)) {
+			/* SNTP set the clock — arm suppression + drift envelope. */
+			observer_mesh.noteTrustedTimeSync();
+		}
 		if (ev & MESH_EVENT_STATUS) {
 			observer_mesh.publishStatus("online");
+			observer_mesh.timeSyncTick();
 		}
 		if (ev & MESH_EVENT_MQTT_CONNECT) {
 			observer_mesh.publishStatus("online");
diff --git a/zephcore/helpers/BaseChatMesh.cpp b/zephcore/helpers/BaseChatMesh.cpp
index e8f283c..f0e4042 100644
--- a/zephcore/helpers/BaseChatMesh.cpp
+++ b/zephcore/helpers/BaseChatMesh.cpp
@@ -139,6 +139,8 @@ void BaseChatMesh::onAdvertRecv(mesh::Packet *packet, const mesh::Identity &id,
 {
 	LOG_DBG("onAdvertRecv: timestamp=%u app_data_len=%u", timestamp, (unsigned)app_data_len);
 
+	onAdvertTimeSample(id, timestamp, packet->getPathHashCount());
+
 	AdvertDataParser parser(app_data, app_data_len);
 	if (!(parser.isValid() && parser.hasName())) {
 		LOG_WRN("onAdvertRecv: invalid parser or no name (valid=%d, hasName=%d)",
diff --git a/zephcore/helpers/BaseChatMesh.h b/zephcore/helpers/BaseChatMesh.h
index 610472b..33b439c 100644
--- a/zephcore/helpers/BaseChatMesh.h
+++ b/zephcore/helpers/BaseChatMesh.h
@@ -157,6 +157,11 @@ protected:
 	virtual void onLoginSent(const ContactInfo &contact) {}
 	virtual void onChannelAdded(ChannelDetails *ch) {}
 
+	/* Every signature-verified advert, before contact filtering/dedup —
+	 * mesh time-sync harvesting hook. */
+	virtual void onAdvertTimeSample(const mesh::Identity &id, uint32_t timestamp,
+		uint8_t hops) { (void)id; (void)timestamp; (void)hops; }
+
 	// Storage concepts for subclasses to override
 	virtual int getBlobByKey(const uint8_t key[], int key_len, uint8_t dest_buf[]) { return 0; }
 	virtual bool putBlobByKey(const uint8_t key[], int key_len, const uint8_t src_buf[], int len) { return false; }
diff --git a/zephcore/helpers/CommonCLI.cpp b/zephcore/helpers/CommonCLI.cpp
index 1372d4e..41f757b 100644
--- a/zephcore/helpers/CommonCLI.cpp
+++ b/zephcore/helpers/CommonCLI.cpp
@@ -5,6 +5,7 @@
 
 #include "CommonCLI.h"
 #include "battery_curve.h"
+#include <helpers/MeshTimeSync.h>
 #include <helpers/time_sync.h>
 #include <adapters/clock/ZephyrRTCDiscover.h>
 #include <helpers/TxtDataHelpers.h>
@@ -119,6 +120,7 @@ void CommonCLI::loadPrefs(const char* path) {
     ok = ok && prefs_read(&file, &_prefs->apc_margin, sizeof(_prefs->apc_margin));           // 293
     ok = ok && prefs_read(&file, &_prefs->flood_max_unscoped, sizeof(_prefs->flood_max_unscoped)); // 294
     ok = ok && prefs_read(&file, &_prefs->flood_max_advert, sizeof(_prefs->flood_max_advert)); // 295
+    ok = ok && prefs_read(&file, &_prefs->meshtimesync, sizeof(_prefs->meshtimesync));         // 296
 
     if (!ok) {
         LOG_WRN("Prefs file %s truncated, some fields use defaults", path);
@@ -164,6 +166,7 @@ void CommonCLI::loadPrefs(const char* path) {
     _prefs->apc_margin = constrain(_prefs->apc_margin, (uint8_t)6, (uint8_t)30);
     _prefs->flood_max_unscoped = constrain(_prefs->flood_max_unscoped, (uint8_t)0, (uint8_t)64);
     _prefs->flood_max_advert = constrain(_prefs->flood_max_advert, (uint8_t)0, (uint8_t)64);
+    _prefs->meshtimesync = constrain(_prefs->meshtimesync, (uint8_t)0, (uint8_t)1);
 
     LOG_INF("Loaded prefs from %s", path);
 }
@@ -231,6 +234,7 @@ void CommonCLI::savePrefs(const char* path) {
     fs_write(&file, &_prefs->apc_margin, sizeof(_prefs->apc_margin));
     fs_write(&file, &_prefs->flood_max_unscoped, sizeof(_prefs->flood_max_unscoped));
     fs_write(&file, &_prefs->flood_max_advert, sizeof(_prefs->flood_max_advert));
+    fs_write(&file, &_prefs->meshtimesync, sizeof(_prefs->meshtimesync));
 
     fs_close(&file);
     LOG_INF("Saved prefs to %s", path);
@@ -339,6 +343,8 @@ void CommonCLI::handleCommand(uint32_t sender_timestamp, const char* command, ch
             getRTCClock()->setCurrentTime(sender_timestamp + 1);
             time_sync_report(TIME_SYNC_CLI);
             zephcore_rtc_save(sender_timestamp + 1);  /* persist to hardware RTC */
+            MeshTimeSync* ts = _callbacks->getMeshTimeSync();
+            if (ts) ts->noteManualSync((uint32_t)(k_uptime_get() / 1000));
             uint32_t now = getRTCClock()->getCurrentTime();
             time_t t = (time_t)now;
             struct tm *tm = gmtime(&t);
@@ -360,6 +366,8 @@ void CommonCLI::handleCommand(uint32_t sender_timestamp, const char* command, ch
             getRTCClock()->setCurrentTime(secs);
             time_sync_report(TIME_SYNC_CLI);
             zephcore_rtc_save(secs);  /* persist to hardware RTC */
+            MeshTimeSync* ts = _callbacks->getMeshTimeSync();
+            if (ts) ts->noteManualSync((uint32_t)(k_uptime_get() / 1000));
             time_t t = (time_t)secs;
             struct tm *tm = gmtime(&t);
             snprintf(reply, CLI_REPLY_SIZE, "OK - clock set: %02d:%02d - %d/%d/%d UTC",
@@ -538,6 +546,19 @@ void CommonCLI::handleCommand(uint32_t sender_timestamp, const char* command, ch
         } else if (memcmp(config, "dc.restarts", 11) == 0) {
             snprintf(reply, CLI_REPLY_SIZE, "> %u",
                      (uint32_t)_callbacks->getDutyCycleTimeoutRestarts());
+        } else if (memcmp(config, "meshtimesync", 12) == 0) {
+            MeshTimeSync* ts = _callbacks->getMeshTimeSync();
+            if (ts == nullptr) {
+                strcpy(reply, "not available");
+            } else {
+                /* Remote replies ride in a ~160-byte packet buffer; only the
+                 * local USB CLI (sender_timestamp == 0) gets the full evidence
+                 * table. */
+                size_t cap = (sender_timestamp == 0) ? CLI_REPLY_SIZE : 158;
+                ts->formatStatus(reply, cap, getRTCClock()->getCurrentTime(),
+                                 (uint32_t)(k_uptime_get() / 1000),
+                                 _prefs->meshtimesync != 0);
+            }
         } else {
             snprintf(reply, CLI_REPLY_SIZE, "??: %s", config);
         }
@@ -948,6 +969,21 @@ void CommonCLI::handleCommand(uint32_t sender_timestamp, const char* command, ch
                 if (val == 0) strcpy(reply, "OK - gps duty=0 (always on)");
                 else snprintf(reply, CLI_REPLY_SIZE, "OK - gps duty=%u s", (unsigned)val);
             }
+        } else if (memcmp(config, "meshtimesync ", 13) == 0) {
+            const char* arg = &config[13];
+            if (_callbacks->getMeshTimeSync() == nullptr) {
+                strcpy(reply, "not available");
+            } else if (memcmp(arg, "on", 2) == 0) {
+                _prefs->meshtimesync = 1;
+                savePrefs();
+                strcpy(reply, "OK - meshtimesync on");
+            } else if (memcmp(arg, "off", 3) == 0) {
+                _prefs->meshtimesync = 0;
+                savePrefs();
+                strcpy(reply, "OK - meshtimesync off");
+            } else {
+                strcpy(reply, "Error: must be on or off");
+            }
         } else {
             snprintf(reply, CLI_REPLY_SIZE, "unknown config: %s", config);
         }
diff --git a/zephcore/helpers/CommonCLI.h b/zephcore/helpers/CommonCLI.h
index 6892064..99ad211 100644
--- a/zephcore/helpers/CommonCLI.h
+++ b/zephcore/helpers/CommonCLI.h
@@ -12,6 +12,8 @@
 #include <helpers/ClientACL.h>
 #include "NodePrefs.h"
 
+class MeshTimeSync;
+
 /* CLI reply buffer size — callers must provide at least this many bytes */
 #define CLI_REPLY_SIZE 256
 
@@ -74,6 +76,9 @@ public:
     virtual uint8_t getAPCTargetMargin() const { return 16; }
     virtual void setAPCTargetMargin(uint8_t margin_db) { (void)margin_db; }
 
+    // Mesh time sync (all roles wire one; nullptr = not compiled/available)
+    virtual MeshTimeSync* getMeshTimeSync() { return nullptr; }
+
     // Sensor manager interface (for GPS)
     virtual double getNodeLat() const { return 0.0; }
     virtual double getNodeLon() const { return 0.0; }
diff --git a/zephcore/helpers/MeshTimeSync.cpp b/zephcore/helpers/MeshTimeSync.cpp
new file mode 100644
index 0000000..e2c741e
--- /dev/null
+++ b/zephcore/helpers/MeshTimeSync.cpp
@@ -0,0 +1,432 @@
+/*
+ * SPDX-License-Identifier: MIT
+ * MeshTimeSync - mesh clock consensus from Ed25519-signed advert timestamps.
+ * See MeshTimeSync.h for the role contract, ARCHITECTURE.md for the design.
+ */
+
+#include "MeshTimeSync.h"
+
+#include <stdio.h>
+#include <string.h>
+
+void MeshTimeSync::reset(uint32_t build_epoch)
+{
+	memset(_slots, 0, sizeof(_slots));
+	_build_epoch = build_epoch;
+	_next_eval_uptime = 0;
+	_suppress_uptime = 0;
+	_suppressed = false;
+	_pedigree_uptime = 0;
+	_pedigree = false;
+	_last_step_uptime = 0;
+	_stepped_once = false;
+	_evals = _abstains = _steps = _bootstrap_steps = _backward_skips = 0;
+	_last_step_delta = 0;
+	_last_step_wall = 0;
+}
+
+MeshTimeSync::Slot *MeshTimeSync::findSlot(const uint8_t *prefix)
+{
+	for (int i = 0; i < MESHTIMESYNC_TABLE_SIZE; i++) {
+		if (_slots[i].used && memcmp(_slots[i].prefix, prefix, sizeof(_slots[i].prefix)) == 0) {
+			return &_slots[i];
+		}
+	}
+	return nullptr;
+}
+
+bool MeshTimeSync::slotTenured(const Slot &s, uint32_t uptime_secs) const
+{
+	return (uptime_secs - s.first_uptime) >= TENURE_SECS &&
+	       s.count >= TENURE_MIN_ADVERTS;
+}
+
+bool MeshTimeSync::slotEligible(const Slot &s, uint32_t uptime_secs) const
+{
+	return s.used && slotTenured(s, uptime_secs) &&
+	       (uptime_secs - s.arrival_uptime) <= MAX_SAMPLE_AGE_SECS;
+}
+
+int64_t MeshTimeSync::slotSkew(const Slot &s, uint32_t local_time, uint32_t uptime_secs) const
+{
+	/* Project the sender's clock forward by our elapsed uptime, then compare
+	 * with our wall clock. Uptime anchor keeps this valid across own steps. */
+	return (int64_t)s.advert_ts + (int64_t)(uptime_secs - s.arrival_uptime) -
+	       (int64_t)local_time;
+}
+
+void MeshTimeSync::onAdvertHeard(const uint8_t *pubkey, uint32_t advert_ts,
+                                 uint8_t hops, uint32_t uptime_secs)
+{
+	if (hops > HOP_CAP) return;
+
+	Slot *s = findSlot(pubkey);
+	if (s) {
+		if (advert_ts <= s->advert_ts) return;  /* per-sender monotonic dedup */
+
+		uint32_t d_up = uptime_secs - s->arrival_uptime;
+		uint32_t d_ts = advert_ts - s->advert_ts;
+		int64_t err = (int64_t)d_ts - (int64_t)d_up;
+		int64_t tol = CONSISTENCY_BASE_SECS + ((int64_t)d_up * CONSISTENCY_PPM) / 1000000;
+		if (err > tol || err < -tol) {
+			/* Sender rebooted, corrected itself, or is lying — re-earn
+			 * tenure from scratch. >150 ppm is beyond crystal physics. */
+			s->first_uptime = uptime_secs;
+			s->count = 1;
+		} else if (s->count < 0xFFFF) {
+			s->count++;
+		}
+		s->advert_ts = advert_ts;
+		s->arrival_uptime = uptime_secs;
+		s->hops = hops;
+		return;
+	}
+
+	for (int i = 0; i < MESHTIMESYNC_TABLE_SIZE; i++) {
+		if (!_slots[i].used) { s = &_slots[i]; break; }
+	}
+
+	if (!s) {
+		/* Hop-priority admission: a new sender may only displace a young
+		 * (not tenure-eligible) entry farther than it; mature entries are
+		 * protected unless silent > 24 h. Among candidates evict farthest,
+		 * then lowest count, then stalest. (Naive LRU churns hub nodes to
+		 * zero eligible voters.) */
+		for (int i = 0; i < MESHTIMESYNC_TABLE_SIZE; i++) {
+			Slot &c = _slots[i];
+			bool young = !slotTenured(c, uptime_secs);
+			bool silent = (uptime_secs - c.arrival_uptime) > MATURE_SILENT_EVICT_SECS;
+			if (!young && !silent) continue;
+			if (c.hops <= hops) continue;
+			if (s == nullptr ||
+			    c.hops > s->hops ||
+			    (c.hops == s->hops && c.count < s->count) ||
+			    (c.hops == s->hops && c.count == s->count &&
+			     c.arrival_uptime < s->arrival_uptime)) {
+				s = &c;
+			}
+		}
+		if (!s) return;  /* table full of closer/protected senders — drop */
+	}
+
+	memcpy(s->prefix, pubkey, sizeof(s->prefix));
+	s->advert_ts = advert_ts;
+	s->arrival_uptime = uptime_secs;
+	s->first_uptime = uptime_secs;
+	s->count = 1;
+	s->hops = hops;
+	s->used = 1;
+}
+
+MeshTimeSync::Consensus MeshTimeSync::computeConsensus(uint32_t local_time,
+                                                       uint32_t uptime_secs,
+                                                       bool bootstrap) const
+{
+	Consensus c;
+	memset(&c, 0, sizeof(c));
+
+	/* Collect eligible votes. Bootstrap relaxes tenure: any sender with a
+	 * fresh sample may vote (the table is RAM-only, so after the reboot that
+	 * dead-ended the clock everything in it is freshly heard anyway). */
+	int64_t skews[MESHTIMESYNC_TABLE_SIZE];
+	int32_t radii[MESHTIMESYNC_TABLE_SIZE];
+	int n = 0;
+	for (int i = 0; i < MESHTIMESYNC_TABLE_SIZE; i++) {
+		const Slot &s = _slots[i];
+		if (!s.used) continue;
+		if ((uptime_secs - s.arrival_uptime) > MAX_SAMPLE_AGE_SECS) continue;
+		if (!bootstrap && !slotTenured(s, uptime_secs)) continue;
+		skews[n] = slotSkew(s, local_time, uptime_secs);
+		radii[n] = RADIUS_BASE_SECS + RADIUS_PER_HOP_SECS * s.hops;
+		n++;
+	}
+	c.eligible = (uint8_t)n;
+	if (n == 0) return c;
+
+	/* Marzullo endpoint sweep — interval intersection with the most votes.
+	 * No absolute outlier thresholds against our own clock: clustering does
+	 * the rejection, so an epoch-0 local clock still finds the true cluster. */
+	int64_t val[2 * MESHTIMESYNC_TABLE_SIZE];
+	int8_t typ[2 * MESHTIMESYNC_TABLE_SIZE];  /* +1 = start, -1 = end */
+	int m = 0;
+	for (int i = 0; i < n; i++) {
+		val[m] = skews[i] - radii[i]; typ[m] = 1;  m++;
+		val[m] = skews[i] + radii[i]; typ[m] = -1; m++;
+	}
+	/* Insertion sort by value; starts before ends at equal values so
+	 * touching intervals count as overlapping. */
+	for (int i = 1; i < m; i++) {
+		int64_t v = val[i];
+		int8_t t = typ[i];
+		int j = i - 1;
+		while (j >= 0 && (val[j] > v || (val[j] == v && typ[j] < t))) {
+			val[j + 1] = val[j];
+			typ[j + 1] = typ[j];
+			j--;
+		}
+		val[j + 1] = v;
+		typ[j + 1] = t;
+	}
+
+	int cur = 0, best = 0, best_idx = 0;
+	for (int i = 0; i < m; i++) {
+		cur += typ[i];
+		if (typ[i] > 0 && cur > best) {
+			best = cur;
+			best_idx = i;
+		}
+	}
+	int64_t lo = val[best_idx];
+	int64_t hi = val[best_idx + 1];  /* next endpoint always exists and, at the
+	                                  * maximum, is an end — a start would have
+	                                  * raised the count past `best` */
+
+	c.valid = true;
+	c.votes_for = (uint8_t)best;
+	c.votes_against = (uint8_t)(n - best);
+	c.mid = (lo + hi) / 2;
+	c.radius = (int32_t)((hi - lo) / 2);
+	return c;
+}
+
+MeshTimeSync::Verdict MeshTimeSync::evaluateNow(uint32_t local_time,
+                                                uint32_t uptime_secs) const
+{
+	Verdict v;
+	memset(&v, 0, sizeof(v));
+
+	bool boot = isBootstrap(local_time);
+	v.bootstrap = boot;
+	v.consensus = computeConsensus(local_time, uptime_secs, boot);
+
+	if (!v.consensus.valid) {
+		v.type = VERDICT_ABSTAIN;
+		v.reason = REASON_NO_DATA;
+		return v;
+	}
+
+	if (boot) {
+		if (v.consensus.votes_for < BOOTSTRAP_QUORUM) {
+			v.type = VERDICT_ABSTAIN;
+			v.reason = REASON_NO_QUORUM;
+			return v;
+		}
+		/* Step to the cluster's low edge: from below, all later refinement
+		 * is forward steps, which are always monotonicity-safe. */
+		int64_t delta = v.consensus.mid - RADIUS_BASE_SECS;
+		if (delta < STEP_TRIGGER_SECS) {
+			v.type = VERDICT_NONE;
+			v.reason = REASON_IN_BAND;
+			return v;
+		}
+		if (isSuppressed(uptime_secs)) {  /* suppression gates bootstrap too */
+			v.type = VERDICT_NONE;
+			v.reason = REASON_SUPPRESSED;
+			return v;
+		}
+		if (_stepped_once && (uptime_secs - _last_step_uptime) < STEP_INTERVAL_SECS) {
+			v.type = VERDICT_NONE;
+			v.reason = REASON_RATE_LIMITED;
+			return v;
+		}
+		v.type = VERDICT_STEP;
+		v.delta = delta;  /* bootstrap exempt from the ±1 h cap */
+		return v;
+	}
+
+	if (v.consensus.eligible < MESHTIMESYNC_QUORUM) {
+		v.type = VERDICT_ABSTAIN;
+		v.reason = REASON_NO_QUORUM;
+		return v;
+	}
+	if ((int)v.consensus.votes_for * 2 <= (int)v.consensus.eligible) {
+		v.type = VERDICT_ABSTAIN;
+		v.reason = REASON_NO_MAJORITY;
+		return v;
+	}
+
+	int64_t mag = v.consensus.mid < 0 ? -v.consensus.mid : v.consensus.mid;
+	if (mag < STEP_TRIGGER_SECS) {
+		v.type = VERDICT_NONE;
+		v.reason = REASON_IN_BAND;
+		return v;
+	}
+	if (isSuppressed(uptime_secs)) {
+		v.type = VERDICT_NONE;
+		v.reason = REASON_SUPPRESSED;
+		return v;
+	}
+	if (_stepped_once && (uptime_secs - _last_step_uptime) < STEP_INTERVAL_SECS) {
+		v.type = VERDICT_NONE;
+		v.reason = REASON_RATE_LIMITED;
+		return v;
+	}
+	if (_pedigree) {
+		/* Physics veto: with a trusted sync + continuous uptime since, a
+		 * real crystal cannot have drifted further than 300 ppm allows. */
+		int64_t envelope = ((int64_t)(uptime_secs - _pedigree_uptime) * PEDIGREE_PPM) / 1000000 +
+		                   PEDIGREE_BASE_SECS;
+		if (mag > envelope) {
+			v.type = VERDICT_ABSTAIN;
+			v.reason = REASON_PEDIGREE;
+			return v;
+		}
+	}
+
+	int64_t delta = v.consensus.mid;
+	if (delta > STEP_CAP_SECS) delta = STEP_CAP_SECS;
+	if (delta < -STEP_CAP_SECS) delta = -STEP_CAP_SECS;
+	v.type = VERDICT_STEP;
+	v.delta = delta;
+	return v;
+}
+
+MeshTimeSync::Verdict MeshTimeSync::tick(uint32_t local_time, uint32_t uptime_secs)
+{
+	Verdict v;
+	if (_next_eval_uptime == 0 || uptime_secs < _next_eval_uptime) {
+		/* First call arms the pacing timer (skips a junk no-data abstain
+		 * at boot); later calls no-op between evaluations. */
+		if (_next_eval_uptime == 0) {
+			_next_eval_uptime = uptime_secs + EVAL_INTERVAL_SECS;
+		}
+		memset(&v, 0, sizeof(v));
+		return v;
+	}
+	_next_eval_uptime = uptime_secs + EVAL_INTERVAL_SECS;
+	_evals++;
+
+	v = evaluateNow(local_time, uptime_secs);
+	if (v.type == VERDICT_ABSTAIN) _abstains++;
+	return v;
+}
+
+void MeshTimeSync::noteManualSync(uint32_t uptime_secs)
+{
+	_suppress_uptime = uptime_secs;
+	_suppressed = true;
+	_pedigree_uptime = uptime_secs;
+	_pedigree = true;
+}
+
+void MeshTimeSync::noteGPSSync(uint32_t uptime_secs)
+{
+	_pedigree_uptime = uptime_secs;
+	_pedigree = true;
+}
+
+void MeshTimeSync::noteStepApplied(int64_t delta, uint32_t local_time,
+                                   uint32_t uptime_secs, bool bootstrap)
+{
+	_last_step_uptime = uptime_secs;
+	_stepped_once = true;
+	_steps++;
+	if (bootstrap) _bootstrap_steps++;
+	_last_step_delta = delta;
+	_last_step_wall = local_time;
+}
+
+bool MeshTimeSync::isSuppressed(uint32_t uptime_secs) const
+{
+	return _suppressed && (uptime_secs - _suppress_uptime) < SUPPRESS_SECS;
+}
+
+uint32_t MeshTimeSync::suppressRemaining(uint32_t uptime_secs) const
+{
+	if (!isSuppressed(uptime_secs)) return 0;
+	return SUPPRESS_SECS - (uptime_secs - _suppress_uptime);
+}
+
+const char *MeshTimeSync::reasonStr(Reason r)
+{
+	switch (r) {
+	case REASON_IN_BAND:      return "in-band";
+	case REASON_NO_DATA:      return "no-data";
+	case REASON_NO_QUORUM:    return "no-quorum";
+	case REASON_NO_MAJORITY:  return "no-majority";
+	case REASON_SUPPRESSED:   return "suppressed";
+	case REASON_RATE_LIMITED: return "rate-limited";
+	case REASON_PEDIGREE:     return "pedigree-veto";
+	default:                  return "-";
+	}
+}
+
+/* Clamp an int64 skew to a printable long (display only). */
+static long clampl(int64_t v)
+{
+	if (v > 2000000000LL) return 2000000000L;
+	if (v < -2000000000LL) return -2000000000L;
+	return (long)v;
+}
+
+int MeshTimeSync::formatStatus(char *out, size_t cap, uint32_t local_time,
+                               uint32_t uptime_secs, bool enabled) const
+{
+	Verdict v = evaluateNow(local_time, uptime_secs);
+	const Consensus &c = v.consensus;
+
+	char verdict[40];
+	if (v.type == VERDICT_STEP) {
+		snprintf(verdict, sizeof(verdict), "step%+ld%s", clampl(v.delta),
+		         v.bootstrap ? " (bootstrap)" : "");
+	} else if (v.type == VERDICT_NONE && v.reason == REASON_IN_BAND) {
+		int64_t mag = c.mid < 0 ? -c.mid : c.mid;
+		snprintf(verdict, sizeof(verdict), "%s",
+		         mag <= DEAD_BAND_SECS ? "ok" : "in-band");
+	} else if (v.type == VERDICT_NONE) {
+		snprintf(verdict, sizeof(verdict), "hold (%s)", reasonStr(v.reason));
+	} else {
+		snprintf(verdict, sizeof(verdict), "abstain (%s)", reasonStr(v.reason));
+	}
+
+	int pos = snprintf(out, cap,
+	                   "%s%s eligible=%u votes=%u/%u skew=%+lds r=%lds -> %s",
+	                   enabled ? "on" : "off",
+	                   enabled ? "" : " (dry-run)",
+	                   (unsigned)c.eligible, (unsigned)c.votes_for,
+	                   (unsigned)c.votes_against,
+	                   c.valid ? clampl(c.mid) : 0L,
+	                   c.valid ? (long)c.radius : 0L,
+	                   verdict);
+	if (pos < 0 || (size_t)pos >= cap) goto full;
+
+	if (_steps > 0) {
+		pos += snprintf(out + pos, cap - pos, "; steps=%lu last=%+lds",
+		                (unsigned long)_steps, clampl(_last_step_delta));
+		if (pos < 0 || (size_t)pos >= cap) goto full;
+	}
+	if (_backward_skips > 0) {
+		pos += snprintf(out + pos, cap - pos, " backskip=%lu",
+		                (unsigned long)_backward_skips);
+		if (pos < 0 || (size_t)pos >= cap) goto full;
+	}
+	if (isSuppressed(uptime_secs)) {
+		pos += snprintf(out + pos, cap - pos, " sup=%luh",
+		                (unsigned long)(suppressRemaining(uptime_secs) / 3600));
+		if (pos < 0 || (size_t)pos >= cap) goto full;
+	}
+	pos += snprintf(out + pos, cap - pos, " evals=%lu abst=%lu",
+	                (unsigned long)_evals, (unsigned long)_abstains);
+	if (pos < 0 || (size_t)pos >= cap) goto full;
+
+	/* Evidence table: one entry per used slot, as many as fit. */
+	for (int i = 0; i < MESHTIMESYNC_TABLE_SIZE; i++) {
+		const Slot &s = _slots[i];
+		if (!s.used) continue;
+		int w = snprintf(out + pos, cap - pos, "\r\n %02x%02x h%u n%u %+lds%s",
+		                 s.prefix[0], s.prefix[1], (unsigned)s.hops,
+		                 (unsigned)(s.count > 99 ? 99 : s.count),
+		                 clampl(slotSkew(s, local_time, uptime_secs)),
+		                 slotEligible(s, uptime_secs) ? " E" : "");
+		if (w < 0 || (size_t)(pos + w) >= cap) {
+			out[pos] = 0;  /* drop the partial entry */
+			return pos;
+		}
+		pos += w;
+	}
+	return pos;
+
+full:
+	out[cap - 1] = 0;
+	return (int)(cap - 1);
+}
diff --git a/zephcore/helpers/MeshTimeSync.h b/zephcore/helpers/MeshTimeSync.h
new file mode 100644
index 0000000..51e88cc
--- /dev/null
+++ b/zephcore/helpers/MeshTimeSync.h
@@ -0,0 +1,193 @@
+/*
+ * SPDX-License-Identifier: MIT
+ * MeshTimeSync - mesh clock consensus from Ed25519-signed advert timestamps.
+ *
+ * Role-agnostic estimator: owns no clock. Each role feeds it signature-
+ * verified adverts (onAdvertHeard), calls tick() from its loop, and applies
+ * STEP verdicts under its own step policy (repeater/observer: bidirectional;
+ * room server/companion: forward-only; GPS-synced boards: sense only).
+ * ZephCore-only divergence from Arduino MeshCore — design rationale in
+ * ARCHITECTURE.md, user-facing doc in MESHTIMESYNC.md.
+ *
+ * All policy timers anchor on uptime, never wall clock, so the very steps
+ * they govern cannot distort them.
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <stddef.h>
+
+#ifdef CONFIG_ZEPHCORE_TIMESYNC_TABLE_SIZE
+  #define MESHTIMESYNC_TABLE_SIZE  CONFIG_ZEPHCORE_TIMESYNC_TABLE_SIZE
+#else
+  #define MESHTIMESYNC_TABLE_SIZE  32
+#endif
+
+#ifdef CONFIG_ZEPHCORE_TIMESYNC_QUORUM
+  #define MESHTIMESYNC_QUORUM  CONFIG_ZEPHCORE_TIMESYNC_QUORUM
+#else
+  #define MESHTIMESYNC_QUORUM  6
+#endif
+
+#ifndef FIRMWARE_BUILD_EPOCH
+  /* Injected by CMakeLists.txt (build-time UNIX epoch, the "provably dead
+   * clock" floor). 0 disables bootstrap mode entirely. */
+  #define FIRMWARE_BUILD_EPOCH 0u
+#endif
+
+class MeshTimeSync {
+public:
+	static constexpr uint8_t  HOP_CAP                  = 3;
+	static constexpr int32_t  RADIUS_BASE_SECS         = 150;
+	static constexpr int32_t  RADIUS_PER_HOP_SECS      = 15;
+	static constexpr uint32_t TENURE_SECS              = 60 * 60;
+	static constexpr uint16_t TENURE_MIN_ADVERTS       = 2;
+	static constexpr uint32_t MAX_SAMPLE_AGE_SECS      = 5 * 24 * 3600;
+	static constexpr uint32_t MATURE_SILENT_EVICT_SECS = 24 * 3600;
+	static constexpr int64_t  CONSISTENCY_BASE_SECS    = 45;
+	static constexpr int64_t  CONSISTENCY_PPM          = 150;
+	static constexpr uint32_t EVAL_INTERVAL_SECS       = 15 * 60;
+	static constexpr int64_t  DEAD_BAND_SECS           = 5 * 60;
+	static constexpr int64_t  STEP_TRIGGER_SECS        = 10 * 60;
+	static constexpr int64_t  STEP_CAP_SECS            = 3600;
+	static constexpr uint32_t STEP_INTERVAL_SECS       = 6 * 3600;
+	static constexpr uint32_t SUPPRESS_SECS            = 7 * 24 * 3600;
+	static constexpr int64_t  PEDIGREE_PPM             = 300;
+	static constexpr int64_t  PEDIGREE_BASE_SECS       = 10 * 60;
+	static constexpr uint8_t  BOOTSTRAP_QUORUM         = 3;
+
+	/* 8-byte prefix is a security floor, not a tuning knob: it is the
+	 * sender's identity for tenure/votes while signatures verify the full
+	 * key, so a shorter prefix lets an attacker grind keypairs to collide
+	 * with a tenured honest voter and reset its tenure with validly-signed
+	 * adverts. If RAM ever matters, cut slot count instead. */
+	struct Slot {
+		uint8_t  prefix[8];
+		uint32_t advert_ts;        /* latest advert timestamp = the vote */
+		uint32_t arrival_uptime;   /* monotonic anchor: skew is recomputed at
+		                            * evaluate time, so a local clock step
+		                            * never stales stored samples */
+		uint32_t first_uptime;     /* tenure start */
+		uint16_t count;            /* adverts this tenure */
+		uint8_t  hops;             /* precision hint only, never a trust signal */
+		uint8_t  used;
+	};
+
+	enum VerdictType : uint8_t { VERDICT_NONE = 0, VERDICT_ABSTAIN, VERDICT_STEP };
+
+	enum Reason : uint8_t {
+		REASON_NONE = 0,
+		REASON_IN_BAND,       /* |skew| below the step trigger */
+		REASON_NO_DATA,       /* no eligible voters */
+		REASON_NO_QUORUM,
+		REASON_NO_MAJORITY,
+		REASON_SUPPRESSED,    /* manual clock set less than 7 days ago */
+		REASON_RATE_LIMITED,  /* < 6 h since last applied step */
+		REASON_PEDIGREE,      /* drift-envelope physics veto */
+	};
+
+	struct Consensus {
+		bool valid;            /* >= 1 eligible vote, intersection computed */
+		uint8_t eligible;
+		uint8_t votes_for;     /* votes inside the best intersection */
+		uint8_t votes_against;
+		int64_t mid;           /* consensus skew midpoint (+ = our clock is behind) */
+		int32_t radius;        /* intersection half-width */
+	};
+
+	struct Verdict {
+		VerdictType type;
+		Reason reason;
+		int64_t delta;         /* seconds to add to the clock (STEP only) */
+		bool bootstrap;
+		Consensus consensus;
+	};
+
+	explicit MeshTimeSync(uint32_t build_epoch = 0) { reset(build_epoch); }
+
+	void reset(uint32_t build_epoch);
+
+	/* Feed one signature-verified advert. hops = flood path length (0 = heard
+	 * direct). Samples beyond HOP_CAP are dropped. */
+	void onAdvertHeard(const uint8_t *pubkey, uint32_t advert_ts, uint8_t hops,
+	                   uint32_t uptime_secs);
+
+	/* Paced evaluation — returns VERDICT_NONE/REASON_NONE unless
+	 * EVAL_INTERVAL_SECS elapsed since the last real evaluation. The caller
+	 * applies STEP verdicts under its role policy and reports the outcome
+	 * via noteStepApplied() (the step rate limit counts applied steps only). */
+	Verdict tick(uint32_t local_time, uint32_t uptime_secs);
+
+	/* Unpaced consensus computation (CLI dry-run view). */
+	Consensus computeConsensus(uint32_t local_time, uint32_t uptime_secs,
+	                           bool bootstrap) const;
+
+	/* Unpaced full policy evaluation (no counter/pacing side effects) —
+	 * what tick() would decide right now. Used by the CLI dry-run. */
+	Verdict evaluateNow(uint32_t local_time, uint32_t uptime_secs) const;
+
+	bool isBootstrap(uint32_t local_time) const { return local_time < _build_epoch; }
+
+	/* Manual clock set (CLI time/clock sync, app time set): arms the 7-day
+	 * suppression window AND drift-envelope pedigree. Suppression gates
+	 * bootstrap too. */
+	void noteManualSync(uint32_t uptime_secs);
+	/* GPS time sync: arms pedigree only (stepping is gated off by the role
+	 * whenever GPS is available and enabled, so no suppression needed). */
+	void noteGPSSync(uint32_t uptime_secs);
+	/* Report an applied step (rate-limit anchor + counters). local_time is
+	 * the wall clock AFTER the step (display only). */
+	void noteStepApplied(int64_t delta, uint32_t local_time, uint32_t uptime_secs,
+	                     bool bootstrap);
+	/* Forward-only roles report a refused backward verdict. */
+	void noteBackwardSkipped() { _backward_skips++; }
+
+	bool isSuppressed(uint32_t uptime_secs) const;
+	uint32_t suppressRemaining(uint32_t uptime_secs) const;
+
+	/* Table access for the CLI evidence dump. */
+	static int tableSize() { return MESHTIMESYNC_TABLE_SIZE; }
+	const Slot &slotAt(int i) const { return _slots[i]; }
+	bool slotEligible(const Slot &s, uint32_t uptime_secs) const;
+	int64_t slotSkew(const Slot &s, uint32_t local_time, uint32_t uptime_secs) const;
+
+	/* Counters for CLI/stats. */
+	uint32_t evalCount() const { return _evals; }
+	uint32_t abstainCount() const { return _abstains; }
+	uint32_t stepCount() const { return _steps; }
+	uint32_t bootstrapStepCount() const { return _bootstrap_steps; }
+	uint32_t backwardSkipCount() const { return _backward_skips; }
+	int64_t lastStepDelta() const { return _last_step_delta; }
+	uint32_t lastStepWall() const { return _last_step_wall; }
+	bool hasStepped() const { return _stepped_once; }
+
+	/* Compact status + evidence-table formatter shared by all role CLIs.
+	 * Writes at most `cap` bytes (NUL-terminated), summary first, then as
+	 * many per-sender entries as fit. */
+	int formatStatus(char *out, size_t cap, uint32_t local_time,
+	                 uint32_t uptime_secs, bool enabled) const;
+
+	static const char *reasonStr(Reason r);
+
+private:
+	Slot *findSlot(const uint8_t *prefix);
+	bool slotTenured(const Slot &s, uint32_t uptime_secs) const;
+
+	Slot _slots[MESHTIMESYNC_TABLE_SIZE];
+	uint32_t _build_epoch;
+
+	/* Policy state — uptime-anchored (see header comment). */
+	uint32_t _next_eval_uptime;
+	uint32_t _suppress_uptime;
+	bool _suppressed;
+	uint32_t _pedigree_uptime;
+	bool _pedigree;
+	uint32_t _last_step_uptime;
+	bool _stepped_once;
+
+	/* Counters. */
+	uint32_t _evals, _abstains, _steps, _bootstrap_steps, _backward_skips;
+	int64_t _last_step_delta;
+	uint32_t _last_step_wall;
+};
diff --git a/zephcore/helpers/NodePrefs.h b/zephcore/helpers/NodePrefs.h
index 6745c76..9a15a17 100644
--- a/zephcore/helpers/NodePrefs.h
+++ b/zephcore/helpers/NodePrefs.h
@@ -64,6 +64,7 @@ struct NodePrefs {
 	uint8_t rx_duty_cycle;          // 1 = RX duty cycle, 0 = continuous RX
 	uint8_t apc_enabled;            // 1 = APC on, 0 = fixed TX power
 	uint8_t apc_margin;             // APC target link margin dB (6-30)
+	uint8_t meshtimesync;           // 1 = mesh time-sync clock correction on (default off)
 
 	/* ---- Companion-only fields ---- */
 	uint8_t manual_add_contacts;
diff --git a/zephcore/helpers/time_sync.h b/zephcore/helpers/time_sync.h
index 2c52f90..e758f28 100644
--- a/zephcore/helpers/time_sync.h
+++ b/zephcore/helpers/time_sync.h
@@ -17,6 +17,7 @@ enum time_sync_source {
 	TIME_SYNC_APP,
 	TIME_SYNC_WIFI,
 	TIME_SYNC_CLI,
+	TIME_SYNC_MESH,   /* mesh time-sync consensus step */
 };
 
 #if IS_ENABLED(CONFIG_ZEPHCORE_UI_DESIGN_JOYSTICK) || \
diff --git a/zephcore/helpers/ui-button/ui_pages.c b/zephcore/helpers/ui-button/ui_pages.c
index 78c4afc..e473d38 100644
--- a/zephcore/helpers/ui-button/ui_pages.c
+++ b/zephcore/helpers/ui-button/ui_pages.c
@@ -133,6 +133,7 @@ static char time_source_tag(enum time_sync_source src)
 	case TIME_SYNC_GPS:  return 'G';  /* GPS fix */
 	case TIME_SYNC_APP:  return 'A';  /* phone/companion app */
 	case TIME_SYNC_WIFI: return 'N';  /* network (SNTP) */
+	case TIME_SYNC_MESH: return 'M';  /* mesh time-sync consensus */
 	default:             return 'L';  /* local: manual/CLI or stale/none */
 	}
 }
diff --git a/zephcore/helpers/ui-joystick/time_sync.c b/zephcore/helpers/ui-joystick/time_sync.c
index 7873d69..93f4d8f 100644
--- a/zephcore/helpers/ui-joystick/time_sync.c
+++ b/zephcore/helpers/ui-joystick/time_sync.c
@@ -17,6 +17,7 @@ static const char *source_short_name(enum time_sync_source src)
 	case TIME_SYNC_APP:  return "App";
 	case TIME_SYNC_WIFI: return "WiFi";
 	case TIME_SYNC_CLI:  return "CLI";
+	case TIME_SYNC_MESH: return "Mesh";
 	default:             return NULL;
 	}
 }
diff --git a/zephcore/src/main_companion.cpp b/zephcore/src/main_companion.cpp
index 1314999..477c930 100644
--- a/zephcore/src/main_companion.cpp
+++ b/zephcore/src/main_companion.cpp
@@ -525,6 +525,12 @@ static void mesh_event_loop(void)
 
 			mesh_housekeeping_ui_refresh();
 
+			/* Mesh time-sync paced evaluation — loop() only runs on
+			 * packet-driven events, so drive the 15-min tick here. */
+			if (companion_mesh_ptr) {
+				companion_mesh_ptr->timeSyncTick();
+			}
+
 			/* Low-battery auto-shutdown (companion only). Self-throttled
 			 * and compiled out unless the board sets a threshold — no
 			 * extra poll, just a cheap call on the existing tick. */
@@ -546,6 +552,12 @@ static void mesh_event_loop(void)
 		 * write here on the main thread instead. */
 		if (events & MESH_EVENT_RTC_SAVE) {
 			zephcore_rtc_save((uint32_t)atomic_get(&pending_rtc_epoch));
+#ifdef ZEPHCORE_LORA
+			/* GPS just set the clock — arm the mesh time-sync drift envelope. */
+			if (companion_mesh_ptr) {
+				companion_mesh_ptr->noteGPSTimeSync();
+			}
+#endif
 		}
 
 #if IS_ENABLED(CONFIG_ZEPHCORE_UI_DESIGN_JOYSTICK)
@@ -703,6 +715,10 @@ public:
 				  int timeout_mins) override {
 		(void)freq; (void)bw; (void)sf; (void)cr; (void)timeout_mins;
 	}
+
+	MeshTimeSync* getMeshTimeSync() override {
+		return companion_mesh.getMeshTimeSync();
+	}
 };
 
 static CompanionCLICallbacks companion_cli_cbs;
diff --git a/zephcore/src/main_repeater.cpp b/zephcore/src/main_repeater.cpp
index f7af7f7..a06d6b9 100644
--- a/zephcore/src/main_repeater.cpp
+++ b/zephcore/src/main_repeater.cpp
@@ -447,6 +447,12 @@ static void repeater_event_loop(void)
 		 * write here on the main thread instead. */
 		if (events & MESH_EVENT_RTC_SAVE) {
 			zephcore_rtc_save((uint32_t)atomic_get(&pending_rtc_epoch));
+#ifdef ZEPHCORE_LORA
+			/* GPS just set the clock — arm the mesh time-sync drift envelope. */
+			if (repeater_mesh_ptr) {
+				repeater_mesh_ptr->noteGPSTimeSync();
+			}
+#endif
 		}
 	}
 }
diff --git a/zephcore/src/main_room_server.cpp b/zephcore/src/main_room_server.cpp
index 7f7eea5..dbbb12f 100644
--- a/zephcore/src/main_room_server.cpp
+++ b/zephcore/src/main_room_server.cpp
@@ -465,6 +465,12 @@ static void room_event_loop(void)
 		 * write here on the main thread instead. */
 		if (events & MESH_EVENT_RTC_SAVE) {
 			zephcore_rtc_save((uint32_t)atomic_get(&pending_rtc_epoch));
+#ifdef ZEPHCORE_LORA
+			/* GPS just set the clock — arm the mesh time-sync drift envelope. */
+			if (room_mesh_ptr) {
+				room_mesh_ptr->noteGPSTimeSync();
+			}
+#endif
 		}
 	}
 }