[tz] Rules for TZ+ database

Paul Eggert eggert at cs.ucla.edu
Thu Sep 5 19:22:53 UTC 2013

Lester Caine wrote:
> how do I ensure that normalization of 1970 data will match 1969 stuff?

By not discarding the imported data.  If the imported data
says the equivalent of "London time", then it should be
stored that way, not as GMT.  That way, if the time zone
tables change, the imported data will be treated
consistently with any further data that are imported later.

It's unwise to require that time stamp conversion tables be
the same on all platforms, and that they never change.
That's not feasible for most nontrivial applications.  Even
if the tz database were perfect and complete for all past
time stamps, such a requirement would still go awry with
time stamps in the future: if someone now schedules a
meeting for October 1, 13:00, Casablanca time, and Morocco
changes its rules between now and then, software will mess
up after the rule change if it blithely relies on UTC
conversions that were computed before the change.

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