easy question: when dst transition occurs at 00:00 hours...

Alex LIVINGSTON alex at agsm.edu.au
Wed Aug 16 00:39:38 UTC 2000

At 09:24 -0700 2000-08-15, Thomas Carey wrote:
>many locales undergo DST transition at midnight - for instance - on Thu 21
>Sept 2000 19:30 - DST will end in Tehran, and local time will go from
>00:00->23:00 (actually, from 23:59 to 23:00).

If "00:00 to 23:00" seems odd, you can say "24:00 to 23:00" - the 
change doesn't take a minute, it happens instantaneously. But just 
what time is it at that instant? Is that defined anywhere?

>My question is whether this
>extra hour occurs (locally) on thursday or friday. I'd assume thursday, as
>00:00 doesn't arrive before the transition.
>can anyone confirm?

I can't confirm, but to me it would seem far stranger and more 
confusing to have  periods from 23:00 to 24:00 both at the beginning 
and the end of the same day, than to have two lots of 23:00 to 24:00 
consecutively within the same day. (I'm ignoring the question of the 
inclusivity or otherwise of the beginning and end instants of these 

Macintosh and Lotus Notes Support / Information Technology (IT)
Australian Graduate School of Management (AGSM)
UNSW SYDNEY NSW 2052 / Australia

Facsimile: +61 2 9931-9349 / Telephone: +61 2 9931-9264
Time     : UTC+11---[last Mar. Sun.---UTC+10---[last Aug. Sun.---UTC+11---

At midday today, Wednesday, August 16,
   time since epoch (1-1-1 at 00:00:00) is
     730347.5 days = 1999.62353779 average Gregorian years
   time until 3rd millennium, 21st century, 201st decade, 2001st year is
     137.5 days = .37646221 average Gregorian years

More information about the tz mailing list