diff --git a/zephcore/adapters/clock/ZephyrRTCDiscover.c b/zephcore/adapters/clock/ZephyrRTCDiscover.c
index 59ecf13..7f0b6b5 100644
--- a/zephcore/adapters/clock/ZephyrRTCDiscover.c
+++ b/zephcore/adapters/clock/ZephyrRTCDiscover.c
@@ -3,7 +3,7 @@
  *
  * Compact raw-I2C hardware-RTC auto-discovery. See ZephyrRTCDiscover.h.
  *
- * Register layouts (sec/min/hour/.../month/year, all BCD) and the per-chip
+ * Register layouts (sec/min/hour/.../month/year) and the per-chip
  * power-loss flags are carried in devicetree via the "zephcore,rtc-i2c"
  * binding, so this reader is generic — adding a new chip is a DT node, not
  * code. Maps were taken from Zephyr's own drivers (rtc_pcf8563.c,
@@ -39,6 +39,7 @@ struct rtc_desc {
 	uint8_t  status_reg;   /* power-loss flag register, or RTC_STATUS_IN_SECONDS */
 	uint8_t  status_mask;  /* "time unreliable" bit within status_reg */
 	const uint8_t *zero;   /* 7 bytes of bits the data sheet shows as 0, or NULL */
+	bool     week_one_hot; /* weekday as one bit per day, not 0-6 */
 #if RTC_RV3028_CFG
 	const uint8_t *cfg;    /* rv3028-eeprom-config triplets, or NULL */
 	uint8_t  cfg_len;
@@ -89,6 +90,7 @@ DT_FOREACH_STATUS_OKAY(RTC_COMPAT, RTC_CFG_ARRAY)
 		.zero        = COND_CODE_1(                           \
 			DT_NODE_HAS_PROP(node, zero_mask),             \
 			(RTC_ZERO_NAME(node)), (NULL)),                \
+		.week_one_hot = DT_PROP(node, weekday_one_hot),       \
 		RTC_CFG_FIELDS(node)                                  \
 		.name        = DT_NODE_FULL_NAME(node),               \
 	},
@@ -227,7 +229,7 @@ static enum rtc_verdict rtc_identify(const struct rtc_desc *d, uint8_t blk[7])
 	return RTC_FOUND;
 }
 
-/* The 7-byte BCD time block for an epoch, in d's register order. */
+/* The 7-byte time block for an epoch, in d's register order. */
 static void rtc_time_block(const struct rtc_desc *d, uint32_t epoch, uint8_t blk[7])
 {
 	int y;
@@ -244,7 +246,7 @@ static void rtc_time_block(const struct rtc_desc *d, uint32_t epoch, uint8_t blk
 	blk[2] = BIN2BCD(hour);
 	/* weekday occupies whichever of index 3/4 the date doesn't. */
 	blk[d->date_index] = BIN2BCD(day);
-	blk[d->date_index == 4 ? 3 : 4] = (uint8_t)dow;
+	blk[d->date_index == 4 ? 3 : 4] = d->week_one_hot ? (uint8_t)BIT(dow) : (uint8_t)dow;
 	blk[5] = BIN2BCD(m);
 	blk[6] = BIN2BCD((unsigned)(y % 100));
 }
diff --git a/zephcore/boards/common/rtc-i2c.dtsi b/zephcore/boards/common/rtc-i2c.dtsi
index b4edf1c..2dabd13 100644
--- a/zephcore/boards/common/rtc-i2c.dtsi
+++ b/zephcore/boards/common/rtc-i2c.dtsi
@@ -78,4 +78,6 @@ rtc_rx8130ce: rtc-rx8130ce@32 {
 	status-mask = <0x02>;  /* VLF — voltage low */
 	/* RX8130CE Application Manual ETM50E-10, 13.2.1 Table 12, p. 22 */
 	zero-mask = [80 80 c0 80 c0 e0 00];
+	/* 14.1.2 Table 15, p. 27: Sunday 01h ... Saturday 40h */
+	weekday-one-hot;
 };
diff --git a/zephcore/boards/nrf52840/meshtracker_x1/meshtracker_x1_nrf52840.dts b/zephcore/boards/nrf52840/meshtracker_x1/meshtracker_x1_nrf52840.dts
index 514578e..f9e07a0 100644
--- a/zephcore/boards/nrf52840/meshtracker_x1/meshtracker_x1_nrf52840.dts
+++ b/zephcore/boards/nrf52840/meshtracker_x1/meshtracker_x1_nrf52840.dts
@@ -354,6 +354,8 @@
 		date-index = <4>;
 		status-reg = <0x0e>;
 		status-mask = <0x02>;
+		/* RX8900 Application Manual ETM45E-11, 8.2.2 Table 15, p. 16 */
+		weekday-one-hot;
 	};
 };
 
diff --git a/zephcore/dts/bindings/rtc/zephcore,rtc-i2c.yaml b/zephcore/dts/bindings/rtc/zephcore,rtc-i2c.yaml
index f241ecc..efde767 100644
--- a/zephcore/dts/bindings/rtc/zephcore,rtc-i2c.yaml
+++ b/zephcore/dts/bindings/rtc/zephcore,rtc-i2c.yaml
@@ -6,9 +6,12 @@
 # the properties below. This keeps the boot-time RTC restore compact
 # (~2-3 KB total, "Path B") instead of pulling in the full RTC subsystem.
 #
-# All supported chips use a 7-byte BCD time block (sec, min, hour, then
-# weekday/date, month, year) starting at "time-reg". Only the day-of-month
-# byte offset varies between parts, captured by "date-index".
+# All supported chips use a 7-byte time block (sec, min, hour, then
+# weekday/date, month, year) starting at "time-reg", BCD in every field but
+# the weekday. The day-of-month byte offset varies between parts, captured
+# by "date-index". The weekday is written as a number 0-6 (Sunday 0), as
+# Zephyr's DS3231, PCF8563 and RV3028 drivers write it, or as one bit per
+# day where "weekday-one-hot" is set.
 
 description: ZephCore data-only I2C RTC descriptor (raw-I2C, no Zephyr RTC driver)
 
@@ -20,7 +23,7 @@ properties:
   time-reg:
     type: int
     required: true
-    description: Register address of the seconds byte (start of the 7-byte BCD block).
+    description: Register address of the seconds byte (start of the 7-byte time block).
 
   date-index:
     type: int
@@ -52,6 +55,13 @@ properties:
       this RTC and is never adopted. Leave it out for a chip whose unused
       bits are not documented as reading 0.
 
+  weekday-one-hot:
+    type: boolean
+    description: >
+      The weekday register holds one bit per day (Sunday bit 0 to Saturday
+      bit 6) rather than a number 0-6. Set it for a chip whose data sheet
+      says so, such as the RX8130CE.
+
   rv3028-eeprom-config:
     type: uint8-array
     description: >