Announcement Bits
Several bits of the WWVB time code give warning of upcoming events.
Bit 55, when set, indicates that the current year is a leap year and will include February 29. This lets a receiver implement the full Gregorian calendar leap-year rules even though the time code does not include the century.
When a leap second is scheduled for the end of a month, bit 56 is set near the beginning of the month, and reset immediately after the leap second insertion.
The DST status bits indicate United States daylight saving time rules. The bits are updated daily during the minute starting at 00:00 UTC. The first DST bit, transmitted at 57 seconds past the minute, changes at the beginning of the UTC day that DST comes into effect or ends. The other DST bit, at second 58, changes 24-hours later (after the DST change). Therefore, if the DST bits differ, DST is changing at 02:00 local time during the current UTC day. Before the next 02:00 local time after that, the bits will be the same.
Each change in the DST bits will first be received in the mainland United States between 16:00 (PST) and 20:00 (EDT), depending on local time zone and on whether DST is about to begin or end. A receiver in the Eastern time zone (UTC−5) must therefore correctly receive the "DST is changing" indication within a seven hour period before DST begins, and six hours before DST ends, if it is to change the local time display at the correct time. Receivers in the Central, Mountain, and Pacific time zones have one, two, and three more hours of advance notice, respectively.
It is up to the receiving clock to apply the change at the next 02:00 local time if it notices the bits differ. If the receiving clock happens to not receive an update between 00:00 UTC and 02:00 local time the day of the change, it should apply the DST change on the next update after that.
An equivalent definition of the DST status bits is that bit 57 is set if DST will be in effect at 24:00Z, the end of the current UTC day. Bit 58 is set if DST was in effect at 00:00Z, the beginning of the current UTC day.
Read more about this topic: WWVB, Amplitude-modulated Time Code
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