PLS> Doesn't the first proposal have the problem that there is no 2:59:59 EST, the Eastern zone having changed to DST? It really just requires the same caveat as the second one--that they will be on EST even with the rest of the Eastern zone having adopted EDT, for that one hour. [snipping down to the proposals...] Our correspondents have suggested two different ways out of this problem. First, and perhaps simplest, you could change the effective time of the rule from 2:00 a.m. EST to 3:00 a.m. EST. This will result in the following transition: 02:59:59 EST ... and then, one second later,... 03:00:00 CDT Second, if you prefer sticking to a 2:00 a.m. transition, you could append the following sentences: Counties that move from the Eastern to the Central Time Zone will switch directly from EST to CDT, so that their clocks need not change. From 2:00 a.m. CDT to 3:00 a.m. CDT these clocks will use CDT even though the rest of the Central Time Zone will still be using CST. This would result in the following transition: 01:59:59 EST ... and then, one second later,... 02:00:00 CDT