diff --git a/src/rp2_common/pico_aon_timer/aon_timer.c b/src/rp2_common/pico_aon_timer/aon_timer.c
index 01f8ee09c..a4d594f63 100644
--- a/src/rp2_common/pico_aon_timer/aon_timer.c
+++ b/src/rp2_common/pico_aon_timer/aon_timer.c
@@ -14,9 +14,6 @@ static aon_timer_alarm_handler_t aon_timer_alarm_handler;
 #include "hardware/rtc.h"
 #include "pico/util/datetime.h"
 
-static __force_inline bool ts_to_tm(const struct timespec *ts, struct tm *tm) {
-    return pico_localtime_r(&ts->tv_sec, tm) != NULL;
-}
 #elif HAS_POWMAN_TIMER
 #include "hardware/powman.h"
 
@@ -28,6 +25,9 @@ static void powman_timer_irq_handler(void) {
     if (aon_timer_alarm_handler) aon_timer_alarm_handler();
 }
 
+static bool ts_to_tm(const struct timespec *ts, struct tm *tm) {
+    return pico_localtime_r(&ts->tv_sec, tm) != NULL;
+}
 #endif
 
 static bool tm_to_ts(const struct tm *tm, struct timespec *ts) {
@@ -89,8 +89,8 @@ bool aon_timer_get_time_calendar(struct tm *tm) {
     return true;
 #elif HAS_POWMAN_TIMER
     struct timespec ts;
-    bool ok = tm_to_ts(tm, &ts);
-    return ok && aon_timer_get_time(&ts);
+    aon_timer_get_time(&ts);
+    return ts_to_tm(&ts, tm);
 #else
     panic_unsupported();
 #endif
@@ -179,29 +179,36 @@ void aon_timer_start_with_timeofday(void) {
     aon_timer_start(&ts);
 }
 
-void aon_timer_start(const struct timespec *ts) {
+bool aon_timer_start(const struct timespec *ts) {
 #if HAS_RP2040_RTC
     rtc_init();
-    aon_timer_set_time(ts);
+    return aon_timer_set_time(ts);
 #elif HAS_POWMAN_TIMER
     // todo how best to allow different configurations; this should just be the default
     powman_timer_set_1khz_tick_source_xosc();
-    powman_timer_set_ms(timespec_to_ms(ts));
-    powman_timer_start();
+    bool ok = aon_timer_set_time(ts);
+    if (ok) {
+        powman_timer_set_ms(timespec_to_ms(ts));
+        powman_timer_start();
+    }
+    return ok;
 #else
     panic_unsupported();
 #endif
 }
 
-void aon_timer_start_calendar(const struct tm *tm) {
+bool aon_timer_start_calendar(const struct tm *tm) {
 #if HAS_RP2040_RTC
     rtc_init();
-    aon_timer_set_time_calendar(tm);
+    return aon_timer_set_time_calendar(tm);
 #elif HAS_POWMAN_TIMER
     // todo how best to allow different configurations; this should just be the default
     powman_timer_set_1khz_tick_source_xosc();
-    aon_timer_set_time_calendar(tm);
-    powman_timer_start();
+    bool ok = aon_timer_set_time_calendar(tm);
+    if (ok) {
+        powman_timer_start();
+    }
+    return ok;
 #else
     panic_unsupported();
 #endif
diff --git a/src/rp2_common/pico_aon_timer/include/pico/aon_timer.h b/src/rp2_common/pico_aon_timer/include/pico/aon_timer.h
index ebe55ebb7..1a3eb5037 100644
--- a/src/rp2_common/pico_aon_timer/include/pico/aon_timer.h
+++ b/src/rp2_common/pico_aon_timer/include/pico/aon_timer.h
@@ -24,21 +24,34 @@
  * This library uses the Powman Timer on RP2350.
  * \endif
  *
- * This library supports both `aon_timer_xxx_calendar()` methods which take a calendar date/time (as struct tm),
- * and `aon_timer_xxx()` methods which take a time since epoch (as struct timespec)
+ * This library supports both `aon_timer_xxx_calendar()` methods which use a calendar date/time (as struct tm),
+ * and `aon_timer_xxx()` methods which use a linear time value relative an internal reference time (via struct timespec).
  *
  * \if rp2040_specific
- * NOTE: On RP2040 the non date/time methods must convert the struct timespec to a date/time value which is handled via
- * the \ref pico_localtime_r method. By default, this pulls in the C library local_time_r method which can lead to a big increase in binary size.
- * `pico_localtime_r` is weak, so can be overridden if a better/smaller alternative is available, otherwise you might consider
- * using the `aon_timer_xxx_calendar()` variants on RP2040
+ * NOTE: On RP2040 the non 'calendar date/time' methods must convert the linear time value to a calendar date/time internally; these methods are:
+ *
+ * * \ref aon_timer_start_with_timeofday
+ * * \ref aon_timer_start
+ * * \ref aon_timer_set_time
+ * * \ref aon_timer_get_time
+ * * \ref aon_timer_enable_alarm
+ *
+ * This conversion is handled by the \ref pico_localtime_r method. By default, this pulls in the C library `local_time_r` method
+ * which can lead to a big increase in binary size. The default implementation of `pico_localtime_r` is weak, so it can be overridden
+ * if a better/smaller alternative is available, otherwise you might consider the method variants ending in `_calendar()` instead on RP2040.
  * \endif
  *
  * \if rp2350_specific
- * NOTE: On RP2350 the date/time methods must convert the struct tm to a struct timespec value which is handled via
- * the \ref pico_mktime method. By default, this pulls in the C library mktime method which can lead to a big increase in binary size.
- * `pico_mktime` is weak, so can be overridden if a better/smaller alternative is available, otherwise you might consider
- * using the `aon_timer_xxx()` variants on RP2350
+ * NOTE: On RP2350 the 'calendar date/time' methods  must convert the calendar date/time to a linear time value internally; these methods are:
+ *
+ * * \ref aon_timer_start_calendar
+ * * \ref aon_timer_set_time_calendar
+ * * \ref aon_timer_get_time_calendar
+ * * \ref aon_timer_enable_alarm_calendar
+ *
+ * This conversion is handled by the \ref pico_mktime method. By default, this pulls in the C library `mktime` method
+ * which can lead to a big increase in binary size. The default implementation of `pico_mktime` is weak, so it can be overridden
+ * if a better/smaller alternative is available, otherwise you might consider the method variants not ending in `_calendar()` instead on RP2350.
  * \endif
  */
 
@@ -66,6 +79,11 @@ typedef void (*aon_timer_alarm_handler_t)(void);
 
 /**
  * \brief Start the AON timer running using the result from the gettimeofday() function as the current time
+ *
+ * \if rp2040_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2040
+ * \endif
+ *
  * \ingroup pico_aon_timer
  */
 void aon_timer_start_with_timeofday(void);
@@ -73,10 +91,30 @@ void aon_timer_start_with_timeofday(void);
 /**
  * \brief Start the AON timer running using the specified timespec as the current time
  * \ingroup pico_aon_timer
- * \param ts the current time
+ *
+ * \if rp2040_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2040
+ * \endif
+ *
+ * \param ts the time to set as 'now'
+ * \return true on success, false if internal time format conversion failed
+ * \sa aon_timer_start_calendar
  */
-void aon_timer_start(const struct timespec *ts);
-void aon_timer_start_calendar(const struct tm *tm);
+bool aon_timer_start(const struct timespec *ts);
+
+/**
+ * \brief Start the AON timer running using the specified calendar date/time as the current time
+ *
+ * \if rp2350_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2350
+ * \endif
+ *
+ * \ingroup pico_aon_timer
+ * \param tm the calendar date/time to set as 'now'
+ * \return true on success, false if internal time format conversion failed
+ * \sa aon_timer_start
+ */
+bool aon_timer_start_calendar(const struct tm *tm);
 
 /**
  * \brief Stop the AON timer
@@ -85,19 +123,59 @@ void aon_timer_start_calendar(const struct tm *tm);
 void aon_timer_stop(void);
 
 /**
- * \brief Update the current time of the AON timer
+ * \brief Set the current time of the AON timer
  * \ingroup pico_aon_timer
+ *
+ * \if rp2040_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2040
+ * \endif
+ *
  * \param ts the new current time
+ * \return true on success, false if internal time format conversion failed
+ * \sa aon_timer_set_time_calendar
  */
 bool aon_timer_set_time(const struct timespec *ts);
+
+/**
+ * \brief Set the current time of the AON timer to the given calendar date/time
+ * \ingroup pico_aon_timer
+ *
+ * \if rp2350_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2350
+ * \endif
+ *
+ * \param tm the new current time
+ * \return true on success, false if internal time format conversion failed
+ * \sa aon_timer_set_time
+ */
 bool aon_timer_set_time_calendar(const struct tm *tm);
 
 /**
  * \brief Get the current time of the AON timer
  * \ingroup pico_aon_timer
+ *
+ * \if rp2040_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2040
+ * \endif
+ *
  * \param ts out value for the current time
+ * \return true on success, false if internal time format conversion failed
+ * \sa aon_timer_get_time_calendar
  */
 bool aon_timer_get_time(struct timespec *ts);
+
+ /**
+ * \brief Get the current time of the AON timer as a calendar date/time
+ * \ingroup pico_aon_timer
+ *
+ * \if rp2350_specific
+ * See \ref pico_aon_timer for caveats with using this method on RP2350
+ * \endif
+ *
+ * \param tm out value for the current calendar date/time
+ * \return true on success, false if internal time format conversion failed
+ * \sa aon_timer_get_time
+ */
 bool aon_timer_get_time_calendar(struct tm *tm);
 
 /**
@@ -116,16 +194,40 @@ void aon_timer_get_resolution(struct timespec *ts);
  * \endif
  * \if rp2040_specific
  * On RP2040 the alarm will not fire if it is in the past.
+ *
+ * See \ref pico_aon_timer for caveats with using this method on RP2040
  * \endif
  *
  * \param ts the alarm time
  * \param handler a callback to call when the timer fires (can be NULL for wakeup_from_low_power = true)
  * \param wakeup_from_low_power true if the AON timer is to be used to wake up from a DORMANT state
  * \return on success the old handler (or NULL if there was none)
- *         on failure, PICO_ERROR_INVALID_ARG
+ *         or PICO_ERROR_INVALID_ARG if internal time format conversion failed
  * \sa pico_localtime_r
  */
 aon_timer_alarm_handler_t aon_timer_enable_alarm(const struct timespec *ts, aon_timer_alarm_handler_t handler, bool wakeup_from_low_power);
+
+/**
+ * \brief Enable an AON timer alarm for a specified calendar date/time
+ * \ingroup pico_aon_timer
+ *
+ * \if rp2350_specific
+ * On RP2350 the alarm will fire if it is in the past
+ *
+ * See \ref pico_aon_timer for caveats with using this method on RP2350
+ * \endif
+ *
+ * \if rp2040_specific
+ * On RP2040 the alarm will not fire if it is in the past.
+ * \endif
+ *
+ * \param tm the alarm calendar date/time
+ * \param handler a callback to call when the timer fires (can be NULL for wakeup_from_low_power = true)
+ * \param wakeup_from_low_power true if the AON timer is to be used to wake up from a DORMANT state
+ * \return on success the old handler (or NULL if there was none)
+ *         or PICO_ERROR_INVALID_ARG if internal time format conversion failed
+ * \sa pico_localtime_r
+ */
 aon_timer_alarm_handler_t aon_timer_enable_alarm_calendar(const struct tm *tm, aon_timer_alarm_handler_t handler, bool wakeup_from_low_power);
 
 /**