Most Casio atomic timekeeping watches have a preset schedule for which they attempt to synchronize with the atomic clock. This usually occurs at 12:00 AM, 1:00 AM, 2:00 AM, 3:00 AM, 4:00 AM, and 5:00 AM. Normally, if the reception of the time signal is successful, then the watch will synchronize with that signal, and not attempt another reception for the remainder of that day. On the date that Daylight Saving Time begins or ends, it typically changes at 2:00 AM, so the concern is that a successful reception at 12:00 AM or 1:00 AM would cause the watch not to switch over to DST later that day (because it would not synchronize with the later time signals).
The signal that is broadcast from the atomic clock in Fort Collins, Colorado also includes the status of Daylight Saving Time. Within the 24 hour period before the change in the status of DST, the signal also includes a special notice that tells the watch that DST is about to change. When this particular notice is received by the watch, it will attempt to synchronize again at 2:00 AM that day. If it synchronizes and receives the signal again, then it should detect that DST has started or ended, and will update the watch accordingly. If it doesn't receive that signal, then it will retry the time synchronization at 3:00 AM, 4:00 AM, and 5:00 AM.
In order for the watch to correctly update during Daylight Saving Time, the DST feature must be correctly set to AUTO. If the DST is set to either ON or OFF, the watch will receive the signal, but the time will be one hour ahead or behind depending on when Daylight Saving Time begins and ends.