[Datetime-SIG] Timeline arithmetic?

Guido van Rossum guido at python.org
Mon Sep 7 21:04:42 CEST 2015

Again, I can't follow this because I don't recall the definition of model
A. But here's a fundamental difference between a timezone-aware datetime
and a POSIX stamp (apart from epoch, range and precision). The difference
applies only to "political" timezones, which may change offsets or DST
rules. The difference is that an aware datetime says "in timezone Z, when
the local clock says T". If T is in the future, politicians may change the
mapping of T to UTC in Z. However, politics can't change the meaning of a
POSIX timestamp. Even for T in the (distant) past the mapping may still
change, when research finds that the rules for Z were different at some
year in the past than they were presumed. So, to me, an aware datetime
*fundamentally* differs from a POSIX timestamp, and even from a pair
composed of a POSIX timestamp plus a tzinfo object. (POSIX timestamps are
however embeddable in datetimes by using a fixed-offset tzinfo.)

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