[Datetime-SIG] Timeline arithmetic?

Carl Meyer carl at oddbird.net
Mon Sep 7 21:42:40 CEST 2015

On 09/07/2015 01:04 PM, Guido van Rossum wrote:
> 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.)

Yes, that's a great description of the precise difference that I've been
trying to describe. Thanks.

(In an attempt to use totally value-neutral terms, I called the "POSIX
timestamp" model "Model A" and the "clock time plus a timezone" -- what
a Python aware datetime is -- "Model B". That probably just introduced
even more confusion.)


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