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

Chris Barker chris.barker at noaa.gov
Mon Apr 13 19:24:43 CEST 2015

Sorry to be brain dead here, but I'm a bit lost:

On Fri, Apr 10, 2015 at 4:32 PM, Alexander Belopolsky < alexander.belopolsky at gmail.com> wrote:

For any given geographical location, loc, and a moment in time t expressed as UTC time, one can tell what time was shown on a "local clock-tower." This defines a function wall(loc, t). This function is a piece-wise linear function which may have regular or irregular discontinuities.

got it.

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?

Thanks for any clarification,

-Chris

--

Christopher Barker, Ph.D. Oceanographer

Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception

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