[Datetime-SIG] DST explained visually

Alexander Belopolsky alexander.belopolsky at gmail.com
Wed Aug 26 22:06:50 CEST 2015


On Wed, Aug 26, 2015 at 3:00 PM, Guido van Rossum <guido at python.org> wrote:
>
> 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.

I decided to establish the origin of the term "fold" before editing the
PEP.  The credit for introducing the term "fall-backward fold" in computing
goes to Paul Eggert of UCLA who used it in various discussions related to
the C language standard that culminated in a defect report #139 [1].

However, apparently, the idea goes back to 1917 Germany.   As Paul Eggert
explained in private correspondence,

"""
fold=0 and fold=1 is like the longstanding German standard for expressing
times as strings, which uses "A" and "B" to distinguish time stamps that
would otherwise be ambiguous.  If I understand things correctly, normally
fold=0, but you can have fold=1 when time stamps are repeated.  For more
about the German standard, you can start with the Wikipedia description
here: [2]
"""

[1]: http://www.open-std.org/jtc1/sc22/wg14/docs/rr/dr_136.html
[2]:
https://de.wikipedia.org/wiki/Sommerzeit#Offizielle_Regelung_der_Zeitumstellung
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