Dieter Maurer <dieter@...> writes:
Eric Thomas wrote at 2007-8-25 07:18 -0500:
I've been having a site problem that I've tracked down to DateTime. Simply enough, the code snippets below both try to add 31 days to the first day in October. I'm expecting this to result in the first day of November.
Good Example:
from DateTime import DateTime start=DateTime('2005/10/01 01:00:00 GMT-5') print start 2005/10/01 01:00:00 GMT-5 earlyStart=start.earliestTime() print earlyStart 2005/10/01 print earlyStart+31 2005/11/01
Bad Example: (Here's the problem)
from DateTime import DateTime start=DateTime('2005/10/01 01:00:00 US/Central') print start 2005/10/01 01:00:00 US/Central earlyStart=start.earliestTime() print earlyStart 2005/10/01 print earlyStart+31 2005/10/31 23:00:00 US/Central
I've confirmed this occurs with several of the US/ timezones (US/Pacific, US/Alaska, etc)
This looks like a daytime saving problem:
With daytime saving, the clock often is put back for an hour at the last samday in october....
The "DateTime" implementation could avoid this *if* the arithmetic would only accept "int" (then it would be very likely that you only want to modify the day but not hours, minutes and seconds). But the arithmetic accepts "float" and that necessitates some surprises with time discontinuities (such as daytime saving switches).
Can anyone help me figure out why the resulting date is coming up 1 hour short of the expected 2005/11/01 ?
Thanks!
Eric
I've submitted this as a zope bug and I agree that it looks like a daytime saving problem. The problem was tiggered nightly by a Google bot hitting various dates on my site's calendar. Even more interesting is that the seconds, milliseconds, and timezone stored within the DateTime are correct after the addition. So, if I compare the two results from the examples I gave using DateTime.isEqualTo(), the answer is True. This really creates a lot of opportunity for error. To work around this, I've modified the addition so that instead of adding 1 day, it adds 1.5 days. This extra half a day is removed by the theDateTime.earliestTime() call but is enough to overcome the DST addition problem. Ugly hack. Thanks all. - Eric