I had trouble understanding this as well, so I wrote a little paper about it, mostly for my own benefit: http://www.cstdbill.com/tzdb/tz-how-to.html I've mentioned it a couple of other times on this list, and nobody has corrected me yet. 8-) I think that much of the confusion is due to the rules wanting to be /both/ transitions (most of the rule) /and/ steady states (the SAVE and LETTER columns). I give an example in the paper. Zones are definitely steady states. My understanding is that they're in effect during the half-open interval, [previous line's UNTIL, this line's UNTIL). If there's no previous line, the lower bound is the beginning of time; for the last line, which has no UNTIL, the upper bound is the end of time. 8-) Does that make sense? --Bill