movement: add time zone conversion in sunrise/sunset face

2 files changed, 16 insertions, 0 deletions

diff --git a/watch-library/watch/watch_utility.c b/watch-library/watch/watch_utility.c
index 02dbfb25..835076d9 100644
--- a/watch-library/watch/watch_utility.c
+++ b/watch-library/watch/watch_utility.c
@@ -146,6 +146,11 @@ watch_date_time watch_utility_date_time_from_unix_time(uint32_t timestamp, uint3
     return retval;
 }
 
+watch_date_time watch_utility_date_time_convert_zone(watch_date_time date_time, uint32_t origin_utc_offset, uint32_t destination_utc_offset) {
+    uint32_t timestamp = watch_utility_date_time_to_unix_time(date_time, origin_utc_offset);
+    return watch_utility_date_time_from_unix_time(timestamp, destination_utc_offset);
+}
+
 float watch_utility_thermistor_temperature(uint16_t value, bool highside, float b_coefficient, float nominal_temperature, float nominal_resistance, float series_resistance) {
     float reading = (float)value;
 
diff --git a/watch-library/watch/watch_utility.h b/watch-library/watch/watch_utility.h
index 0a8f2b13..66af6ed5 100644
--- a/watch-library/watch/watch_utility.h
+++ b/watch-library/watch/watch_utility.h
@@ -70,6 +70,17 @@ uint32_t watch_utility_date_time_to_unix_time(watch_date_time date_time, uint32_
   */
 watch_date_time watch_utility_date_time_from_unix_time(uint32_t timestamp, uint32_t utc_offset);
 
+/** @brief Converts a time from a given time zone to another time zone.
+  * @param date_time The watch_date_time that you wish to convert
+  * @param origin_utc_offset The number of seconds from UTC in the origin time zone
+  * @param destination_utc_offset The number of seconds from UTC in the destination time zone
+  * @return A watch_date_time for the given UNIX timestamp and UTC offset, or if outside the range that
+  *         watch_date_time can represent, a watch_date_time with all fields set to 0.
+  * @note Adapted from MIT-licensed code from musl, Copyright © 2005-2014 Rich Felker, et al.:
+  *       https://github.com/esmil/musl/blob/1cc81f5cb0df2b66a795ff0c26d7bbc4d16e13c6/src/time/__secs_to_tm.c
+  */
+watch_date_time watch_utility_date_time_convert_zone(watch_date_time date_time, uint32_t origin_utc_offset, uint32_t destination_utc_offset);
+
 /** @brief Returns a temperature in degrees Celsius for a given thermistor voltage divider circuit.
   * @param value The raw analog reading from the thermistor pin (0-65535)
   * @param highside True if the thermistor is connected to VCC and the series resistor is connected