According to the respective documentation, UIApplicationSignificantTimeChange notification is not available on WatchOS. Is there an equivalent notification or way to be notified about significant time changes (new day mainly) on an Apple Watch app?
AFAIK there is no built in equivalent in WatchKit. However, you should be able to create an equivalent using UNCalendarNotificationTrigger and setting the trigger to the next day each time a new notification is delivered.