Re: Date Math Question and Daylight Savings Time
Re: Date Math Question and Daylight Savings Time
- Subject: Re: Date Math Question and Daylight Savings Time
- From: Stephen Todd <email@hidden>
- Date: Thu, 20 Jan 2005 13:39:19 -0700
I recently dealt with this problem myself, and I found a lot of trouble
in using NSTimestamp. The temptation is to call
timestampByAddingGregorianUnits() to modify timestamps. The docs say
that "This method preserves "clock time" across changes in Daylight
Savings Time zones and leap years." While the fact that it accounts
for leap years might be true, it does _not_ take care of Daylight
Saving. We do separate queries from midnight to midnight time periods
across a a week. You would think that you could just
timestampByAddingGregorianUnits() it by adding a day, but in reality,
only 24 hours are added (Oct 31, 2004, for example, has 25 hours).
Well, what you should read between the lines in the docs is that
NSTimestamps are in GMT and doesn't participate in DLS.
The solution that I found that worked is to convert it the timestamp to
a GregorianCalendar, make your changes, and then create a new time
stamp using the info from the GregorianCalendar. It's a pain, but it
seems to work for me. So, what I concluded was that don't do any
calculations or manipulations on NSTimestamps, just use a
GregorianCalendar.
I don't use GregorianCalendars extensively so YMMV, but this seemed to
work for us.
Stephen
On Jan 20, 2005, at 11:20 AM, James Cicenia wrote:
Hello All -
I am doing some date math calculations and during daylight savings
time my calculations seems to be off a day.
Any thoughts on why?
- James Cicenia
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