[issue13466] new timezones

[Marc-Andre Lemburg]

Marc-Andre Lemburg <mal at egenix.com> added the comment:

Amaury Forgeot d'Arc wrote:
> 
> Amaury Forgeot d'Arc <amauryfa at gmail.com> added the comment:
> 
>> A fairly "correct" way is to query the time zone database at time module
>> import time by using the DST and GMT offset of that time.
> 
> But that does not give the *other* timezone :-(

Which other timezone ?

You set time.timezone to the GMT offset of the import time
and then subtract another 3600 seconds in case tm_isdst is
set.

>> IMO time.timezone and time.daylight should be deprecated since they
>> will give wrong results around DST changes (both switch times and
>> legal changes such as the described one) in long running processes
>> such as daemons.
> 
> time.timezone is the non-DST timezone: this value does not change around
> the DST change date. 

No, but time.daylight changes and time.timezone can change in situations
like these where a region decides to change the way DST is dealt
with, e.g. switches to the DST timezone or moves the switchover date.

Since both values are tied to a specific time I don't think it's
a good idea to have them as module globals.

> That's why the current implementation uses "absolute"
> dates like the of January: DST changes are often in March and October.

Such an algorithm can be used as fallback solution in case
tm_isdst is -1 (unknown), but not in case the DST information
is available.

> What about this algorithm:
> - pick the first of January and the first of July surrounding the current date
> - if both have tm_idst==0, the region has no DST. Use the current GMT offset for
>   both timezone and altzone; daylight=0

Those two steps are not necessary. If tm_isdst == 0, you already know that
the current time zone is not DST.

> - otherwise, use the *current* time and get its DST and GMT offset. This is enough
> to compute both timezone and altzone (with the relation altzone=timezone-3600)

That's what I suggested above.

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