datetime issue

[Chris Angelico]

On Mon, Sep 17, 2012 at 1:40 AM,  <pandora.koura at gmail.com> wrote:
> Cant it be written more easily as:
>  date = (datetime.datetime.utcnow(+2)
>
> i know this is not thhe correct syntax but it just needs a way to add GMT+2 hours since utc=gmt

I've dithered about whether to open this can of worms or let sleeping
dogs lie, and I finally decided to make some metaphor potpourri.

Simple UTC offsets are not the same thing as local time. There are a
variety of other considerations, but the main one is Daylight Robbery
Time, where every government wants to put its own stamp on things by
fiddling with the DST rules a bit. Most places will move their clocks
an hour forward for summer and then back for winter. But!

* Some places are sensible and don't have DST at all (eg
Australia/Queensland, America/Arizona)
* Northern hemisphere summer corresponds to southern hemisphere
winter, and vice versa
* Everyone has a different way of defining the boundaries of summer and winter
* And just to confuse us all, Irish Standard Time is used in summer,
and they *subtract* an hour for winter time!

The only way to handle timezones correctly and keep yourself even
marginally sane is to hand the job on to somebody else. I dodged the
whole issue by letting our Postgres database handle everything for us
(date/time formats, timezones, the lot), and having our internal
systems all just work in UTC. You'll likely find it easiest to do the
same thing with a Python library.

Just don't try to pretend to yourself that Greece uses GMT+2, because
it's not that simple. For one thing, it's currently summer there...

ChrisA