Paul & Tim, thank you for your examples and explanations. I'm finally starting to get an understanding of the scope of the complex problem. I've also read some of the 'explanatory' files to better understand the database contents, POSIX format, etc, and I think I'm closer to understanding how to use the database for my clock. It's really interesting that some countries/localities still make short-term changes to DST start/stop dates, and that some areas experience four changes in a year! My first reaction was: "I won't offer my clock to those countries", but then I thought: "But they're the ones who need it even more than me!" So I need to try to deal with those weird rule changes in my clock. My first thought is: "How does the TZ database record rules when they change four times in a year rather than two?" My next problem was "where do I find these countries in the database, since there's no 'MiddleEast' file?" But then I was alerted to Zone1970.tab, and found that they were included in the 'Asia' file. So I went looking for a place where these quadruple rules might apply. My first search was for Baghdad, which turned up another issue in the current (2017b) Asia file that doesn't quite make sense to me. From line 1146... Rule Iraq 1986 1990 - Mar lastSun 1:00s 1:00 D # IATA SSIM (1991/1996) says Apr 1 12:01am UTC; guess the ':01' is a typo. # Shanks & Pottenger say Iraq did not observe DST 1992/1997; ignore this. # Rule Iraq 1991 2007 - Apr 1 3:00s 1:00 D Rule Iraq 1991 2007 - Oct 1 3:00s 0 S # Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Asia/Baghdad 2:57:40 - LMT 1890 2:57:36 - BMT 1918 # Baghdad Mean Time? 3:00 - +03 1982 May 3:00 Iraq +03/+04 Because there is no *current* RULE for Baghdad (the last expired 2007 according to the table), I assume that DST no longer occurs there. But Bill Seymour's explanation of the digits in the FORMAT column are 'standard time name/daylight saving time name'. Given that DST no longer applies (if my Rule interpretation is correct) and the standard GMT offset is +3h, why would these (now identical?) zones be named +03 and +04? At this stage I haven't continued on my search for a quad-change locality (I'll wait till I better understand the 'simple' ones, such as the above, first!), but if anyone can direct me to a quad-change rule, it would save me some time further down the track. Regards, Daniel