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.) Carl
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