[Datetime-SIG] DST explained visually

Guido van Rossum guido at python.org
Wed Aug 26 22:09:58 CEST 2015

On Wed, Aug 26, 2015 at 12:37 PM, Alexander Belopolsky <
alexander.belopolsky at gmail.com> wrote:

> On Wed, Aug 26, 2015 at 3:00 PM, Guido van Rossum <guido at python.org>
> wrote:
>> The proposal in PEP 495 adds a 'fold' flag whose value is 0 *except* for
>> local times mapped from UTC period C-D; between C and D local time is
>> between R and S with fold=1. (Note that the current text of the PEP has a
>> flag named 'first' whose definition is the opposite; but the plan is to
>> switch to fold=0. In any case it's one bit of information and it's only
>> used for times between P-Q.)
> I have edited [1] your sketch to show the UTC mappings of two local times:
> g in the gap and f in the fold: (g, fold=0) maps to G0, (g, fold=1) maps to
> G1, (f, fold=0) maps to F0, and (f, fold=1) maps to F1. Note that G1 < G0
> while F1 > F0.  This may look arbitrary, but it follows from a consistent
> rule: fold=0 is the intersection with the line that is solid (valid) before
> the transition and fold=1 is the intersection with the line that is solid
> (valid) after the transition.
> [1]: https://github.com/abalkin/ltdf/blob/master/dst-visual.jpg

Cool! Which reminds me, there are some edge cases to consider. What's the
local time for UTC=A? And for UTC=C? I guess the rule is to use half-open
intervals on the X axis that are open on the right, so that A maps to Q and
C maps to R.

--Guido van Rossum (python.org/~guido)
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