diff --git a/ports/stm/peripherals/rtc.c b/ports/stm/peripherals/rtc.c
index 15f82ba78dac..7c8cb65e17cd 100644
--- a/ports/stm/peripherals/rtc.c
+++ b/ports/stm/peripherals/rtc.c
@@ -32,7 +32,9 @@
 #include "shared/timeutils/timeutils.h"
 
 // Default period for ticks is 1/1024 second
-#define TICK_DIVISOR 1024
+#define TICKS_PER_SECOND 1024
+// Based on a 32768 kHz clock
+#define SUBTICKS_PER_TICK 32
 
 STATIC RTC_HandleTypeDef hrtc;
 
@@ -47,6 +49,13 @@ volatile uint32_t cached_date = 0;
 volatile uint32_t seconds_to_minute = 0;
 volatile uint32_t cached_hours_minutes = 0;
 
+// The RTC starts at 2000-01-01 when it comes up.
+// If the RTC is set to a later time, the ticks the RTC returns will be offset by the new time.
+// Remember that offset so it can be removed when returning a monotonic tick count.
+static int64_t rtc_ticks_offset;
+// Normalized to be 0-31 inclusive, so always positive.
+static uint8_t rtc_subticks_offset;
+
 volatile bool alarmed_already[2];
 
 bool peripherals_wkup_on = false;
@@ -59,6 +68,9 @@ uint32_t stm32_peripherals_get_rtc_freq(void) {
 }
 
 void stm32_peripherals_rtc_init(void) {
+    rtc_ticks_offset = 0;
+    rtc_subticks_offset = 0;
+
     // RTC oscillator selection is handled in peripherals/<family>/<line>/clocks.c
     __HAL_RCC_RTC_ENABLE();
     hrtc.Instance = RTC;
@@ -74,49 +86,9 @@ void stm32_peripherals_rtc_init(void) {
     HAL_NVIC_EnableIRQ(RTC_Alarm_IRQn);
 }
 
-#if CIRCUITPY_RTC
-void stm32_peripherals_rtc_get_time(timeutils_struct_time_t *tm) {
-    RTC_DateTypeDef date = {0};
-    RTC_TimeTypeDef time = {0};
-
-    int code;
-    if ((code = HAL_RTC_GetTime(&hrtc, &time, RTC_FORMAT_BIN)) == HAL_OK &&
-        (code = HAL_RTC_GetDate(&hrtc, &date, RTC_FORMAT_BIN)) == HAL_OK) {
-        tm->tm_hour = time.Hours;
-        tm->tm_min = time.Minutes;
-        tm->tm_sec = time.Seconds;
-        tm->tm_wday = date.WeekDay - 1;
-        tm->tm_mday = date.Date;
-        tm->tm_mon = date.Month;
-        tm->tm_year = date.Year + 2000;
-        tm->tm_yday = -1;
-    }
-}
-
-void stm32_peripherals_rtc_set_time(timeutils_struct_time_t *tm) {
-    RTC_DateTypeDef date = {0};
-    RTC_TimeTypeDef time = {0};
-
-    time.Hours = tm->tm_hour;
-    time.Minutes = tm->tm_min;
-    time.Seconds = tm->tm_sec;
-    date.WeekDay = tm->tm_wday + 1;
-    date.Date = tm->tm_mday;
-    date.Month = tm->tm_mon;
-    date.Year = tm->tm_year - 2000;
-    time.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
-    time.StoreOperation = RTC_STOREOPERATION_RESET;
-
-    if (HAL_RTC_SetTime(&hrtc, &time, RTC_FORMAT_BIN) != HAL_OK ||
-        HAL_RTC_SetDate(&hrtc, &date, RTC_FORMAT_BIN) != HAL_OK) {
-        // todo - throw an exception
-    }
-}
-#endif
-
 // This function is called often for timing so we cache the seconds elapsed computation based on the
 // register value. The STM HAL always does shifts and conversion if we use it directly.
-uint64_t stm32_peripherals_rtc_raw_ticks(uint8_t *subticks) {
+STATIC uint64_t stm32_peripherals_rtc_raw_ticks(uint8_t *subticks) {
     // Disable IRQs to ensure we read all of the RTC registers as close in time as possible. Read
     // SSR twice to make sure we didn't read across a tick.
     __disable_irq();
@@ -157,13 +129,84 @@ uint64_t stm32_peripherals_rtc_raw_ticks(uint8_t *subticks) {
     uint8_t seconds = (uint8_t)(time & (RTC_TR_ST | RTC_TR_SU));
     seconds = (uint8_t)RTC_Bcd2ToByte(seconds);
     if (subticks != NULL) {
-        *subticks = subseconds % 32;
+        *subticks = subseconds % SUBTICKS_PER_TICK;
     }
 
-    uint64_t raw_ticks = ((uint64_t)TICK_DIVISOR) * (seconds_to_date + seconds_to_minute + seconds) + subseconds / 32;
+    uint64_t raw_ticks = ((uint64_t)TICKS_PER_SECOND) * (seconds_to_date + seconds_to_minute + seconds) + subseconds / SUBTICKS_PER_TICK;
     return raw_ticks;
 }
 
+// This function returns monotonically increasing ticks by adjusting away the RTC tick offset
+// from the last time the date was set.
+uint64_t stm32_peripherals_rtc_monotonic_ticks(uint8_t *subticks) {
+    uint8_t raw_subticks;
+    uint64_t monotonic_ticks = stm32_peripherals_rtc_raw_ticks(&raw_subticks) - rtc_ticks_offset;
+    int8_t monotonic_subticks = raw_subticks - rtc_subticks_offset;
+    // Difference might be negative. Normalize to 0-31.
+    // `while` not really necessary; should only loop 0 or 1 times.
+    while (monotonic_subticks < 0) {
+        monotonic_ticks--;
+        monotonic_subticks += SUBTICKS_PER_TICK;
+    }
+    *subticks = (uint8_t)monotonic_subticks;
+    return monotonic_ticks;
+}
+
+#if CIRCUITPY_RTC
+void stm32_peripherals_rtc_get_time(timeutils_struct_time_t *tm) {
+    RTC_DateTypeDef date = {0};
+    RTC_TimeTypeDef time = {0};
+
+    int code;
+    if ((code = HAL_RTC_GetTime(&hrtc, &time, RTC_FORMAT_BIN)) == HAL_OK &&
+        (code = HAL_RTC_GetDate(&hrtc, &date, RTC_FORMAT_BIN)) == HAL_OK) {
+        tm->tm_hour = time.Hours;
+        tm->tm_min = time.Minutes;
+        tm->tm_sec = time.Seconds;
+        tm->tm_wday = date.WeekDay - 1;
+        tm->tm_mday = date.Date;
+        tm->tm_mon = date.Month;
+        tm->tm_year = date.Year + 2000;
+        tm->tm_yday = -1;
+    }
+}
+
+void stm32_peripherals_rtc_set_time(timeutils_struct_time_t *tm) {
+    RTC_DateTypeDef date = {0};
+    RTC_TimeTypeDef time = {0};
+
+    uint8_t current_monotonic_subticks;
+    uint64_t current_monotonic_ticks = stm32_peripherals_rtc_monotonic_ticks(&current_monotonic_subticks);
+
+    // SubSeconds will always be set to zero.
+    time.Hours = tm->tm_hour;
+    time.Minutes = tm->tm_min;
+    time.Seconds = tm->tm_sec;
+    date.WeekDay = tm->tm_wday + 1;
+    date.Date = tm->tm_mday;
+    date.Month = tm->tm_mon;
+    date.Year = tm->tm_year - 2000;
+    time.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
+    time.StoreOperation = RTC_STOREOPERATION_RESET;
+
+    if (HAL_RTC_SetTime(&hrtc, &time, RTC_FORMAT_BIN) != HAL_OK ||
+        HAL_RTC_SetDate(&hrtc, &date, RTC_FORMAT_BIN) != HAL_OK) {
+        // todo - throw an exception
+    }
+
+    uint8_t raw_subticks;
+    rtc_ticks_offset = stm32_peripherals_rtc_raw_ticks(&raw_subticks) - current_monotonic_ticks;
+    int8_t rtc_subticks_offset_signed = raw_subticks - current_monotonic_subticks;
+    // Difference might be negative. Normalize subticks to 0-31.
+    // `while` not really necessary; should only loop 0 or 1 times.
+    while (rtc_subticks_offset_signed < 0) {
+        rtc_ticks_offset--;
+        rtc_subticks_offset_signed += SUBTICKS_PER_TICK;
+    }
+    rtc_subticks_offset = (uint8_t)rtc_subticks_offset_signed;
+}
+#endif
+
 void stm32_peripherals_rtc_assign_wkup_callback(void (*callback)(void)) {
     wkup_callback = callback;
 }
@@ -177,7 +220,7 @@ void stm32_peripherals_rtc_set_wakeup_mode_seconds(uint32_t seconds) {
 }
 
 void stm32_peripherals_rtc_set_wakeup_mode_tick(void) {
-    HAL_RTCEx_SetWakeUpTimer_IT(&hrtc, (rtc_clock_frequency / 16) / TICK_DIVISOR, RTC_WAKEUPCLOCK_RTCCLK_DIV2);
+    HAL_RTCEx_SetWakeUpTimer_IT(&hrtc, (rtc_clock_frequency / 16) / TICKS_PER_SECOND, RTC_WAKEUPCLOCK_RTCCLK_DIV2);
 }
 
 void stm32_peripherals_rtc_enable_wakeup_timer(void) {
@@ -205,9 +248,9 @@ void stm32_peripherals_rtc_set_alarm(uint8_t alarm_idx, uint32_t ticks) {
     uint64_t raw_ticks = stm32_peripherals_rtc_raw_ticks(NULL) + ticks;
 
     RTC_AlarmTypeDef alarm;
-    if (ticks > TICK_DIVISOR) {
+    if (ticks > TICKS_PER_SECOND) {
         timeutils_struct_time_t tm;
-        timeutils_seconds_since_2000_to_struct_time(raw_ticks / TICK_DIVISOR, &tm);
+        timeutils_seconds_since_2000_to_struct_time(raw_ticks / TICKS_PER_SECOND, &tm);
         alarm.AlarmTime.Hours = tm.tm_hour;
         alarm.AlarmTime.Minutes = tm.tm_min;
         alarm.AlarmTime.Seconds = tm.tm_sec;
@@ -221,7 +264,7 @@ void stm32_peripherals_rtc_set_alarm(uint8_t alarm_idx, uint32_t ticks) {
     }
 
     alarm.AlarmTime.SubSeconds = rtc_clock_frequency - 1 -
-        ((raw_ticks % TICK_DIVISOR) * 32);
+        ((raw_ticks % TICKS_PER_SECOND) * SUBTICKS_PER_TICK);
     if (alarm.AlarmTime.SubSeconds > rtc_clock_frequency) {
         alarm.AlarmTime.SubSeconds = alarm.AlarmTime.SubSeconds +
             rtc_clock_frequency;
diff --git a/ports/stm/peripherals/rtc.h b/ports/stm/peripherals/rtc.h
index 474cba1881cd..93febbb5ccd1 100644
--- a/ports/stm/peripherals/rtc.h
+++ b/ports/stm/peripherals/rtc.h
@@ -36,7 +36,7 @@
 
 uint32_t stm32_peripherals_get_rtc_freq(void);
 void stm32_peripherals_rtc_init(void);
-uint64_t stm32_peripherals_rtc_raw_ticks(uint8_t *subticks);
+uint64_t stm32_peripherals_rtc_monotonic_ticks(uint8_t *subticks);
 
 void stm32_peripherals_rtc_assign_wkup_callback(void (*callback)(void));
 void stm32_peripherals_rtc_set_wakeup_mode_seconds(uint32_t seconds);
diff --git a/ports/stm/supervisor/port.c b/ports/stm/supervisor/port.c
index 1fb3f363774f..0a8d9a5b1902 100644
--- a/ports/stm/supervisor/port.c
+++ b/ports/stm/supervisor/port.c
@@ -385,7 +385,7 @@ __attribute__((used)) void HardFault_Handler(void) {
 }
 
 uint64_t port_get_raw_ticks(uint8_t *subticks) {
-    return stm32_peripherals_rtc_raw_ticks(subticks);
+    return stm32_peripherals_rtc_monotonic_ticks(subticks);
 }
 
 // Enable 1/1024 second tick.
diff --git a/shared-module/time/__init__.c b/shared-module/time/__init__.c
index b4575b18a2cf..7d4f585ea1c9 100644
--- a/shared-module/time/__init__.c
+++ b/shared-module/time/__init__.c
@@ -38,6 +38,8 @@ uint64_t common_hal_time_monotonic_ns(void) {
     uint64_t ticks = port_get_raw_ticks(&subticks);
     // A tick is 976562.5 nanoseconds so multiply it by the base and add half instead of doing float
     // math.
+    // A subtick is 1/32 of a tick.
+    // 30518 is 1e9 / 32768
     return 976562 * ticks + ticks / 2 + 30518 * subticks;
 }