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+# Timestamp-Based Packet Filters
+
+**Issue:** #289  
+**Status:** Draft  
+**Depends on:** #286 (timestamp display config)
+
+## Summary
+
+Extend the existing filter engine (`packet-filter.js`) with a `time` field type supporting absolute ISO timestamps, relative durations, and range expressions. The filter compiles date expressions to epoch milliseconds at parse time so per-packet evaluation is a single numeric comparison — no date parsing in the hot path.
+
+## Syntax
+
+### Absolute (ISO 8601)
+
+```
+time > "2024-01-01T00:00:00Z"
+time <= "2024-06-15"
+time == "2024-03-01"
+```
+
+Quoted strings after `time` are parsed as dates. Partial dates (`"2024-01-01"`) are treated as midnight UTC. All absolute values are interpreted as UTC regardless of the user's display preference.
+
+### Relative
+
+```
+time > 2h ago
+time > 30m ago
+time > 7d ago
+```
+
+The lexer recognizes `<number><unit> ago` as a relative time literal. Supported units: `s` (seconds), `m` (minutes), `h` (hours), `d` (days). At compile time, the relative offset is resolved to an absolute epoch ms value (`Date.now() - offset`). This means a compiled filter's relative thresholds are frozen at compile time — recompile to refresh.
+
+### Shorthand
+
+```
+time.ago < 30m
+time.ago < 2h
+```
+
+`time.ago` resolves to `Date.now() - packet.timestamp`. The comparison value is a duration literal (`30m`, `2h`, `7d`). This is syntactic sugar and semantically equivalent to the relative form but reads more naturally for "show me recent packets."
+
+### Range
+
+```
+time between "2024-01-01" "2024-01-02"
+time between 1h ago 30m ago
+```
+
+`between` is a ternary operator: `field between <low> <high>`. Compiles to `low <= field && field <= high`. Both bounds are inclusive.
+
+### Combinable with existing filters
+
+```
+type == Advert && time > 1h ago
+snr > 5 && time between "2024-01-01" "2024-01-02"
+(type == GRP_TXT || type == TXT_MSG) && time.ago < 30m
+```
+
+## Grammar Extension
+
+### New token types
+
+| Token | Pattern | Example |
+|-------|---------|---------|
+| `DURATION` | `/^\d+[smhd]$/` | `30m`, `2h`, `7d` |
+| `AGO` | keyword `ago` | `ago` |
+| `BETWEEN` | keyword `between` | `between` |
+
+### Lexer changes
+
+1. After reading an identifier that matches `\d+[smhd]`, emit `DURATION` token instead of `FIELD`.
+2. Recognize `ago` and `between` as keywords (like `and`/`or`).
+
+### Parser changes
+
+In `parseComparison()`:
+
+1. **Relative time:** If field is `time` and value tokens are `DURATION AGO`, compute `Date.now() - durationToMs(duration)` and store as a numeric epoch ms value in the AST node.
+2. **Absolute time:** If field is `time` and value is a `STRING`, attempt `new Date(value).getTime()`. If `NaN`, return parse error. Store epoch ms.
+3. **`time.ago` shorthand:** If field is `time.ago`, the value is a `DURATION`. Store the duration in ms. At evaluation, compute `now - packet_ts` and compare against the duration.
+4. **`between`:** If operator token is `BETWEEN`, consume two values (same type resolution as above). Emit `{ type: 'between', field, low, high }`.
+
+### AST node shapes
+
+```js
+// Absolute/relative (pre-resolved to epoch ms)
+{ type: 'comparison', field: 'time', op: '>', value: 1704067200000 }
+
+// time.ago (duration in ms)
+{ type: 'comparison', field: 'time.ago', op: '<', value: 1800000 }
+
+// between (both bounds as epoch ms)
+{ type: 'between', field: 'time', low: 1704067200000, high: 1704153600000 }
+```
+
+## Field Resolution
+
+Add to `resolveField()`:
+
+```js
+if (field === 'time') return packet.timestamp;  // epoch ms
+if (field === 'time.ago') return Date.now() - packet.timestamp;
+```
+
+`packet.timestamp` is the packet's capture time in epoch milliseconds. This field already exists in the data model (populated from the DB `created_at` column).
+
+## Time Semantics
+
+- **Filter expressions:** Always UTC. `"2024-01-01"` means `2024-01-01T00:00:00Z`.
+- **Display:** Follows the user's timestamp config from #286 (UTC/local/relative).
+- **Relative times:** Computed against `Date.now()` at compile time. The compiled filter is a snapshot — if the filter stays active for hours, relative thresholds drift. This is acceptable; filters are typically short-lived or recompiled on interaction.
+
+**No timezone specifiers in the filter syntax.** UTC only. This avoids ambiguity and parsing complexity. Users who think in local time can use the relative syntax (`time > 2h ago`) which is timezone-agnostic.
+
+## Performance
+
+### Compile-time work (once)
+
+- Parse date strings → epoch ms via `new Date().getTime()` (~1μs per date)
+- Parse duration strings → ms via multiplication (~0ns, trivial arithmetic)
+- Relative `ago` → `Date.now() - offset` (~0ns)
+
+### Per-packet evaluation (hot path)
+
+- `time` comparison: one numeric read + one numeric compare. Same cost as `snr > 5`.
+- `time.ago`: one subtraction + one compare. Two arithmetic ops. **Important:** cache `Date.now()` once per filter pass (e.g., in a closure variable set before iterating packets), not per-packet. 30K `Date.now()` calls are ~1ms but it's a pointless syscall tax.
+- `between`: two numeric compares.
+
+**No `Date` objects created per packet. No string parsing per packet. No regex per packet.**
+
+At 30K packets, the time filter adds ~0.1ms total to filter evaluation — dominated by the existing field resolution and AST walk overhead. No measurable regression.
+
+### Implementation note: `between` as sugar
+
+`between` should compile to `{ type: 'and', left: { type: 'comparison', field, op: '>=', value: low }, right: { type: 'comparison', field, op: '<=', value: high } }` — reusing existing comparison evaluation. No new AST node type, no new evaluator branch. The parser desugars it; the evaluator never sees `between`.
+
+### Implementation note: `time.ago` and `Date.now()` caching
+
+The `compile()` function should return a filter that accepts an optional `now` parameter:
+
+```js
+var compiled = compile('time.ago < 30m');
+var now = Date.now();
+packets.filter(function(p) { return compiled.filter(p, now); });
+```
+
+If `now` is not passed, `Date.now()` is called once on the first invocation and reused for the entire filter pass. This avoids 30K syscalls and ensures consistent evaluation within a single pass.
+
+## Carmack Review Notes
+
+Reviewed with a performance-first lens (30K+ packets, real-time updates):
+
+1. **✅ No allocations in hot path.** All date parsing happens at compile time. Per-packet evaluation is pure numeric comparison — same cost as existing `snr > 5` filters.
+
+2. **⚠️ `Date.now()` per-packet for `time.ago`.** Fixed above — cache once per filter pass via optional `now` parameter or closure. Without this, 30K packets × `Date.now()` = ~1ms wasted on a monotonic clock syscall that returns the same value.
+
+3. **✅ `between` as sugar, not a new node type.** Desugar in the parser to reuse existing `and` + `comparison` nodes. Zero new code paths in the evaluator = zero new bugs in the evaluator.
+
+4. **✅ Parser complexity is bounded.** Three new token types, one new keyword. The parser remains LL(1) — no backtracking, no ambiguity. `DURATION AGO` is a clear two-token lookahead only when field is `time`.
+
+5. **✅ Memory impact negligible.** Compiled time filters add one or two floats to the AST. At 16 bytes per node, even complex expressions with multiple time clauses are <100 bytes.
+
+6. **⚠️ Compiled filter staleness for relative times.** Spec acknowledges this. Acceptable for a web UI where filters are recompiled on user interaction. If filters persist across long WebSocket sessions, consider recompiling on a timer (every 60s). This is a future concern, not a blocker.
+
+7. **✅ No regex in hot path.** Duration parsing uses a simple char check on the last character + `parseInt`. Cheaper than any regex.
+
+A compiled time filter adds one or two 64-bit float values to the AST. Negligible — roughly 16 bytes per time comparison node.
+
+## URL Integration
+
+Time filters appear in the URL hash query string like any other filter:
+
+```
+#/packets?filter=time%20%3E%201h%20ago
+#/packets?filter=type%20%3D%3D%20Advert%20%26%26%20time%20%3E%20%222024-01-01%22
+```
+
+The filter text is URL-encoded and round-trips through `encodeURIComponent`/`decodeURIComponent`. No special handling needed — the existing filter-in-URL mechanism (#286 or current) works unchanged.
+
+For convenience, a future milestone could add dedicated `timeFrom`/`timeTo` query params that inject into the filter, but this is not required for the initial implementation.
+
+## Wireshark Compatibility
+
+| Wireshark syntax | CoreScope equivalent | Notes |
+|------------------|---------------------|-------|
+| `frame.time >= "2024-01-01"` | `time >= "2024-01-01"` | We use `time` instead of `frame.time` for brevity. Could alias `frame.time` → `time` later. |
+| `frame.time_relative < 60` | `time.ago < 60s` | Wireshark uses seconds float; we use duration literals |
+| `frame.time_delta` | Not supported | Inter-packet delta is a different feature |
+
+We intentionally diverge from Wireshark where their syntax is verbose or requires pcap-specific concepts. CoreScope's filter language prioritizes brevity and readability for a web UI. A `frame.time` alias for `time` can be added trivially in the field resolver if users request it.
+
+## Milestones
+
+### M1: Core time filtering (parser + evaluator)
+- Add `DURATION`, `AGO`, `BETWEEN` tokens to lexer
+- Extend parser for `time` field special handling
+- Add `time` and `time.ago` to `resolveField()`
+- Implement `between` AST node evaluation
+- Unit tests: absolute, relative, ago, between, combined with existing filters, edge cases (bad dates, invalid units)
+- **Test:** filter 30K packets by time in <50ms (assert in test)
+
+### M2: UI integration
+- Filter bar autocomplete hints for time syntax
+- Help tooltip / cheat sheet update with time examples
+- Verify URL round-trip with time filters
+- Playwright E2E test: enter time filter, verify packet list updates
+
+### M3: Polish
+- `frame.time` alias
+- Error messages for common mistakes ("did you mean `time > 1h ago`?")
+- Consider dedicated time range picker UI widget (out of scope for this spec)
+
+## Testing
+
+### Unit tests (add to `test-packet-filter.js`)
+
+```js
+// Absolute time
+c = compile('time > "2024-01-01"');
+assert(c.filter({ timestamp: new Date('2024-06-01').getTime() }), 'after 2024-01-01');
+assert(!c.filter({ timestamp: new Date('2023-06-01').getTime() }), 'before 2024-01-01');
+
+// Relative time
+c = compile('time > 1h ago');
+assert(c.filter({ timestamp: Date.now() - 30 * 60000 }), '30m ago passes 1h filter');
+assert(!c.filter({ timestamp: Date.now() - 2 * 3600000 }), '2h ago fails 1h filter');
+
+// time.ago shorthand
+c = compile('time.ago < 30m');
+assert(c.filter({ timestamp: Date.now() - 10 * 60000 }), '10m ago < 30m');
+assert(!c.filter({ timestamp: Date.now() - 60 * 60000 }), '60m ago not < 30m');
+
+// between
+c = compile('time between "2024-01-01" "2024-01-02"');
+assert(c.filter({ timestamp: new Date('2024-01-01T12:00:00Z').getTime() }), 'in range');
+assert(!c.filter({ timestamp: new Date('2024-01-03').getTime() }), 'out of range');
+
+// Combined
+c = compile('type == Advert && time > 1h ago');
+assert(c.filter({ payload_type: 4, timestamp: Date.now() - 1000 }), 'combined pass');
+assert(!c.filter({ payload_type: 4, timestamp: Date.now() - 7200000 }), 'combined fail time');
+assert(!c.filter({ payload_type: 1, timestamp: Date.now() - 1000 }), 'combined fail type');
+
+// Error cases
+c = compile('time > "not-a-date"');
+assert(c.error, 'invalid date string');
+
+c = compile('time > 5x ago');
+assert(c.error, 'invalid duration unit');
+
+// Performance
+var start = Date.now();
+c = compile('time > 1h ago && type == Advert');
+var packets = [];
+for (var i = 0; i < 30000; i++) {
+  packets.push({ payload_type: i % 5, timestamp: Date.now() - i * 1000 });
+}
+packets.forEach(function(p) { c.filter(p); });
+assert(Date.now() - start < 50, 'filter 30K packets in <50ms');
+```
+
+### Playwright tests
+
+- Enter `time > 1h ago` in filter bar → verify packet count decreases
+- Enter invalid time filter → verify error message appears
+- Reload page with time filter in URL → verify filter is applied