[Zope3-dev] Re: [Python-Dev] Holes in time

[Guido van Rossum]

> I was working on a possible solution when I stumbled across the fact 
> that the current tzinfo documentation doesn't seem to specify whether 
> the dst() method expects the "dt" argument to be in terms of UTC or 
> local time.  Sometimes when working on the problem I assumed dt was in 
> UTC, making the conversion from UTC to local time easy, and at other 
> times I assumed dt was in local time, making the conversion from local 
> time to UTC easy.  Which is it?

Local time.  This is the source of most problems!

> Once that's decided, it seems like the "hard" case (whichever is the 
> hard one) could be solved by first computing the UTC offset at the time 
> requested, then computing the UTC offset at a time adjusted by the 
> offset.  If the two computed offsets are different, you know you've 
> straddled a daylight savings boundary, and maybe the second offset is 
> the correct offset.  That's just a guess.

You're slowly rediscovering the guts of datetimetz.astimezone().  Have
a look at the python code in
python/nondist/sandbox/datetime/dateyime.py before you go any
further. :-)

--Guido van Rossum (home page: http://www.python.org/~guido/)