On Fri, Aug 10, 2018 at 10:18:19AM -0500, David Wright wrote:
Currently we have a consumer radio clock which is a source of mystery to me twice a year: the DST change occurs in the early evening on Saturday instead of Sunday morning.
The clock isn't properly decoding the DST bits. See https://en.wikipedia.org/wiki/WWVB
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 the 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.
Your clock is probably only looking at the first bit, and skipping the logic for when to actually apply the change. A lot of the functionality in the time code actually allows for relatively graceful degradation like this to facilitate simpler or more complex receivers.
Mike Stone