main: calculate exact number of seconds before sleep time ends

main/clock_main.c

@@ -105,6 +105,10 @@ static void display_time() {
     }
 }
 
+static uint16_t min(uint16_t a, uint16_t b) {
+    return (a < b) ? a : b;
+}
+
 static uint16_t max(uint16_t a, uint16_t b) {
     return (a > b) ? a : b;
 }
@@ -112,7 +116,12 @@ static uint16_t max(uint16_t a, uint16_t b) {
 static uint64_t determine_sleep_time(struct tm *local_time) {
     uint16_t wakeup_time_sec;
     if (!should_enable_display(local_time->tm_hour)) {
-        wakeup_time_sec = HALF_HOUR_IN_SECONDS;
+        if (local_time->tm_hour == 5) {
+            wakeup_time_sec = min((60 - local_time->tm_min) * 60
+                                  + (60 - local_time->tm_sec), HALF_HOUR_IN_SECONDS);
+        } else {
+            wakeup_time_sec = HALF_HOUR_IN_SECONDS;
+        }
     } else {
         wakeup_time_sec = 60 - local_time->tm_sec;
     }