In my earlier ruminations on the changes to Daylight Savings Time I mainly considered computers.  However, the change will affect many other devices besides computers.

All of the clocks that you see in stores that bill themselves as “atomic clocks” are actually regular clocks that set themselves using the NIST WWVB reference signal.  WWVB broadcasts at 60kHz, using a BCD time code format.  I have three clocks that use this method (including my alarm clock) to keep themselves accurate.  Generally, they resync once per day, late at night (when the wave propagation is best for LF transmissions).  These clocks also have adjustments for DST and for your local timezone, so that they correctly compute the correct local time based on the UTC standard that is being sent.

I was initially worried that these clocks will not properly adjust to the new beginning and ending dates of DST.  However, it appears that the time code format includes two bits for signalling DST.  Page 21 of NIST Time and Frequency Services (NIST Special Publication 432) (PDF) gives the information on how these bits signal if DST is in effect or when DST is starting or ending:

Daylight saving time (DST) and standard time (ST) information is transmitted at seconds 57 and 58. When ST is in effect, bits 57 and 58 are set to 0. When DST is in effect, bits 57 and 58 are set to 1. On the day of a change from ST to DST bit 57 changes from 0 to 1 at 0000 UTC, and bit 58 changes from 0 to 1 exactly 24 hours later. On the day of a change from DST back to ST bit 57 changes from 1 to 0 at 0000 UTC, and bit 58 changes from 1 to 0 exactly 24 hours later.

So it appears that the data stream includes enough information for the clocks to adjust to the change as long as NIST sends the DST/ST transition bits on the correct days.  Of course, this also requires that the clock designers followed the standard and didn’t code the DST start/end rules into the firmware.  I’m hoping that they followed the standard.  Otherwise these clocks may require manual fiddling (provided it’s possible) to continue to show the correct time in 2007.

Update:  A short article on problems for VCR’s and cell phones, via Slashdot.