[Python-Dev] Aware datetime from naive local time Was: Status on PEP-431 Timezones (original) (raw)

Alexander Belopolsky alexander.belopolsky at gmail.com
Mon Apr 13 19:45:19 CEST 2015

On Mon, Apr 13, 2015 at 1:24 PM, Chris Barker <chris.barker at noaa.gov> wrote:

Because of these discontinuities, an equation wall(loc, t) = lt may have 0, 1 or 2 solutions. This is where I'm confused -- I can see how going from "wall" time ("local" time, etc) to UTC has 0, 1, or 2 solutions: One solution most of the time Zero solutions when we "spring forward" -- i.e. there is no 2:30 am on March 8, 2015 in the US timezones that use DST Two solutions when we "fall back", i.e. there are two 2:30 am Nov 1, 2015 in the US timezones that use DST But I can't see where there are multiple solutions the other way around -- doesn't a given UTC time map to one and only one "wall time" in a given timezone? Am I wrong, or is this a semantic question as to what "wall" time means?

You are right about what wall() means, but I should have been more explicit about knowns and unknowns in the wall(loc, t) = lt equation.

In that equation I considered loc (the geographical place) and lt (the time on the clock tower) to be known and t (the universal (UTC) time) to be unknown. A solution to the equation is the value of the unknown (t) given the values of the knowns (loc and lt).

The rest of your exposition is correct including "a given UTC time maps to one and only one 'wall time' in a given timezone." However, different UTC times may map to the same wall time and some expressible wall times are not results of a map of any UTC time. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20150413/32083b91/attachment.html>

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