NSCalendarDate, NSTimer - Daylight Saving Time (DST) problem is tricky...
NSCalendarDate, NSTimer - Daylight Saving Time (DST) problem is tricky...
- Subject: NSCalendarDate, NSTimer - Daylight Saving Time (DST) problem is tricky...
- From: Simon Liu <email@hidden>
- Date: Wed, 6 Apr 2005 13:11:27 +0100
Hi,
I'm trying to implement a timer based scheduler. When DST ends, how
does that affect NSTimers? Is there any documentation and specs on
how NSCalendarDate and NSTimers behave when moving from DST to
non-DST?
For example, 30 Oct 05, in Europe/Brussels timezone, at 02:00, clocks
go back as DST ends. So the time is actually like this:
01:00:00 --> 01:59:59 --> 01:00:00 --> 01:59:59 --> 02:00:00
There are "physically" two hours between 01:00 and 02:00. More info
is here: http://www.timeanddate.com/time/aboutdst.html
Say you have a timer set to fire at 01:30:00, since it happens twice,
when should it fire? From some investigation, it seems that with
NSCalendarDate, 01:30:00 only occurs once, so what happened to the
second occurence?
Using NSCalendarDate, I notice that things start happening around
01:59:52. If you set times by using [NSCalendarDate
dateWithYear:month:day:hour:minute:second:timezone:] class method, and
then ask for the -timeIntervalSinceReferenceDate, here's what you get.
timeIntervalSinceReferenceDate --- time set
152323192 --- 30 Oct 05, 01:59:52 Europe/Brussels
152326793 --- 30 Oct 05, 01:59:53 Europe/Brussels *** Notice the
timeInterval jump
152326800 --- 30 Oct 05, 02:00:00 Europe/Brussels
So what happens if we set the date by time interval using
-initWithTimeIntervalSinceReferenceDate:
-setTimeZone:
152323193 --- 30 Oct 05, 01:59:53 Europe/Brussels
152323201 --- 30 Oct 05, 02:00:01 Europe/Brussels *** Why not 01:00:01 ?
152326800 --- 30 Oct 05, 02:00:00 Europe/Brussels
With a time interval of 152323201, the time is actually further in
future than a time interval of 152326800 - how strange. Why does the
time being represented not go back the hour?
Any Apple engineers or other folks out there who can shed some light?
I'm trying to implement a scheduler and any advice or general
guidelines on how to deal with this DST issue of a repeating hour
would be appreciated.
BTW: I haven't discussed the clocks going forward as that seeks a
little clearer. If the clocks go forward at 2am by 1 hour, then the
time jumps from 01:59:59 --> 03:00:00 and any times supposedly from
02:00:00 to 02:59:59 are actually invalid (or should be considered as
such).
Regards,
Simon
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