Make Datetime Timezone Aware From UTC Offset and DST Bit

Question

I am currently battling the cruel beast that is timezone localization in my django application, and having some trouble... I want to make naive datetimes timezone aware, based on a location. I have a database of zip codes that have the UTC offset in hours, as well as a 0 or 1 depending on if the zip codes adhere to DST. How might I use this data to accurately apply a timezone to my datetimes? Ideally the datetime would respond to changes in DST, rather than just always simply following the UTC offset.

Answer 1

With pytz it's not hard to convert the datetimes as you describe; the only complication is getting tzinfo instances corresponding to the time zone descriptions in your database.

The problem is that real timezones are more complicated than just offset + DST. For example, different regions adopted DST at different points in history, and different regions in the world can make the DST switch at different points in the year.

If your usage is only for the US, and only concerns future (not historical) dates, then there are a couple options that should yield accurate results (though note the caveat below):

1. Just create your own concrete tzinfo subclass that uses the offset and DST flag from your database. For example, the Python documentation gives sample code for "a complete implementation of current DST rules for major US time zones."

2. Map from the offset / DST to the corresponding pytz tzinfo object. Since there are only a handful of possible combinations in the US, just figure out which timezone name corresponds and use that.

    TZ_MAP = { ... (-5, 1): pytz.timezone('US/Eastern') ... } tz = TZ_MAP[(offset, is_dst)] 

Once you have the tzinfo instance the conversion is simple, but note that dealing with DST involves inherent ambiguities. For example, when the clock is turned back at 2am, all the times between 1am and 2am occur twice in the local timezone. Assuming you don't know which one you actually mean, you can either pick one arbitrarily, or raise an exception.

# with no is_dst argument, pytz will guess if there is ambiguity
aware_dt = tz.localize(naive_dst)

# with is_dst=None, pytz will raise an exception if there is ambiguity
aware_dt = tz.localize(naive_dst, is_dst=None)