Sorry, I guess I should have searched your answers more carefully. In your first answer, for example, the significant word is "Yes"; the rest I already knew. Since I didn't understand why you'd include all the rest, I wasn't sure that the "Yes" answered the question I was asking. I see now that it did. As for going backwards to find the standard time, I don't think that's valid. Consider the following excerpts from the source for America/Indiana/Vincennes: -5:00 - EST 2006 Apr 2 2:00 -6:00 US C%sT 2007 Nov 4 2:00 After 02:00 on 2006 Apr 2, the standard time should be UTC-6; but if I go to the most recent non-DST entry (the actual previous one in this case), I get UTC-5. And I see that I escaped too early from your third answer as well. 8-) My current understanding is that, if the TZ string is POSIX-style (as evidenced by failure to open a file of that name), and if the TZ string has no rule, then I use the posixrules file; and only then do I worry about the two arrays I'm asking about. In this case, the entries in these arrays tell me how to interpret the values in the array of tzh_timecnt transition times. Is that correct? Thanks, --Bill