How to create a Timezone aware Time object that works with all dates?

Question

I have been struggling with this issue for a while now. I feel that I understand Timezones well, and I think I understand how to use pytz properly so I usually use that and I typically don't have any problems. Maybe I am trying to use the wrong tools for what I want.

With my current application, I need to work in abstract time objects. That is, I care about something that happens at 16:00. I don't care that it happens at 16:00 ON December 23rd. It seems that pytz prefers working with datetime objects: and that makes sense. It can't figure out the offset since it's different based on the day for daylight savings and historical reasons and so on.

I'm trying to allow users to coordinate daily events all around the world. If a user in Japan says the event is from 21:00 to 23:00 every day, I want users in US/Central to see 6:00 to 8:00 every day. And I thought I had this working... until two weeks ago that is. See, DST just ended in most of the United States, so what now is 6:00-8:00 was actually previously 7:00-9:00.

This breaks my idea that I need to typically store times in UTC, then convert them only for viewing. The timezone it was created in is actually very important. If we reverse this, and a user in a US time zone that observes Daylight Savings sets an event time, that time needs to change in Japan even though they don't observe it! If I store that time as UTC, nothing changes in Japan, but that is not the functionality I want.

So I would like to store as a time object with tzinfo. But you can't create a time object with an accurate pytz tzinfo without a date, but the date isn't important. If I use the current date to figure out the tzinfo then that will actually cease to be accurate once that time zone changes.

I guess my question is: what the best way to store "4 PM Eastern" in a way that can be retrieved anywhere in the world, at any time in the future? That includes Eastern! I want it to be 4PM during DST and also outside of DST. I can't store it as UTC, because 12:00 UTC is the same time as 12:00 UTC during the entire year, but I don't want that. What I think I want is, an "abstract" or "temporary" pytz.timezone that doesn't have an actual offset until the date (the date of viewing) is given. Is that a thing? I've read countless questions on this site, both the python and pytz docs, and can't find anything like this, or anyone with a similar problem. Everything seems to talk about specific datetimes or working only within datetimes but that doesn't seem relevant to my issue.

My application is pretty huge so it's hard to pull out the specific parts, but I can try to show what I've tried and why things don't work.

event_time = datetime.time(hour=12, tzinfo=pytz.timezone("US/Eastern")) would be my ideal solution. But using pytz for tzinfo creation is not a good idea (this will give me an offset like -5:04 because of historical reasons) - is there a way to specify the version of US/Eastern to use?

datetime.now(pytz.timezone("US/Eastern")).replace(hour=12, minute=0, second=0, microsecond=0).timetz() gives me something that looks like what I want, but it only functions correctly as long as US/Eastern doesn't change. If I apply this to a date that I have moved back before the DST change, it gives me 13:00, which is not what I want.

Marcel Wilson · Accepted Answer

I built a tzinfo class that automatically handles the conversion of DST. I got the idea from the USTimeZone class example in the datetime documentation.

The trick here is the pytz timezone database has all the historical dates of when daylight savings goes into effect. That's also why when you create a datetime object without a date, it converts the DST incorrectly; it's going based off the first entry in the database.

from datetime import datetime, tzinfo, timedelta
import pytz

ZERO = timedelta(0)

def format_timedelta(td):
    if td < timedelta(0):
        return '-' + format_timedelta(-td)
    else:
        # Change this to format positive timedeltas the way you want
        return str(td)



class WorldTimeZone(tzinfo):
    """
    A self adjusting according to DST rules in the PYTZ database tzinfo class
    See pytz.all_timezones for a list of all zone names and offsets.
    """

    def __init__(self, zone):
        """
        :param zone:  (str) Proper tzdatabase timze zone name. 
        """
        # initialize the pytz timezone with current time
        # this is done to avoid confusing tznames found in the start of the tz database
        # _utcoffset should always be STD rather than DST.
        self.pytzinfo = self.__getSTD(zone)
        self._utcoffset = self.pytzinfo._utcoffset


    @staticmethod
    def __getSTD(tname):
        """
        This returns a pytz timezone object normalized to standard time for the zone requested.
        If the zone does not follow DST or a future transition time cannot be found, it normalizes to NOW instead.

        :param tname:  Proper timezone name found in the tzdatabase. example: "US/Central"
        """

        # This defaults to the STD time for the zone rather than current time which could be DST
        tzone = pytz.timezone(tname)
        NOW = datetime.now(tz=pytz.UTC)
        std_date = NOW
        hasdst = False
        try:
            #transitions are in UTC.  They need to be converted to localtime once we find the correct STD transition.
            for utcdate, info in zip(tzone._utc_transition_times, tzone._transition_info):
                utcdate = utcdate.replace(tzinfo=pytz.UTC)
                utcoffset, dstoffset, tzname = info
                if dstoffset == ZERO:
                    std_date = utcdate
                if utcdate > NOW:
                    hasdst = True
                    break
        except AttributeError:
            std_date = NOW
        if not hasdst:
            std_date = NOW
        std_date = tzone.normalize(std_date)
        return std_date.tzinfo

    # This needs to be dynamic because pytzinfo updates everytime .dst() is called; which is a lot.
    @property
    def _dst(self):
        return self.pytzinfo._dst

    def __repr__(self):
        # return self.pytzinfo.__repr__()
        if self._dst:
            dst = 'DST'
        else:
            dst = 'STD'
        if self._utcoffset > timedelta(seconds=0):
            msg = ''
        else:
            msg = ''
        return msg % (self.pytzinfo.zone, self.pytzinfo._tzname,
                      format_timedelta(self._utcoffset + self._dst), dst)

    def __str__(self):
        return "%s %s" % (self.pytzinfo._tzname, self.pytzinfo)

    def tzname(self, dt):
        # print "   TZNAME called"
        return "%s %s" % (self.pytzinfo._tzname, self.pytzinfo)

    def utcoffset(self, dt):
        # print "   UTCOFFSET CALLED"
        return self._utcoffset + self.dst(dt)

    def dst(self, dt):
        # print "   DST CALLED"
        if dt is None or dt.tzinfo is None:
            # An exception may be sensible here, in one or both cases.
            # It depends on how you want to treat them.  The default
            # fromutc() implementation (called by the default astimezone()
            # implementation) passes a datetime with dt.tzinfo is self.
            return ZERO

        assert dt.tzinfo is self  # WE ASSUME THE TZINFO ON THE DATE PASSED IN IS OUR TZINFO OBJECT.
        tmpdt = self.pytzinfo.normalize(dt)
        self.pytzinfo = tmpdt.tzinfo
        return tmpdt.tzinfo._dst

Sample code

EST = WorldTimeZone('US/Eastern')
PST = WorldTimeZone('US/Pacific')
dt_dst = datetime(2018, 11, 1, 1, 30, 00)
dt_std = datetime(2018, 11, 6, 1, 30, 00)
est_dst = dt_dst.replace(tzinfo=EST)
est_std = dt_std.replace(tzinfo=EST)
pst_dst = est_dst.astimezone(PST)
pst_std = est_std.astimezone(PST)

print(f"{dt_dst} >> {est_dst} >> {pst_dst}")
print(f"{dt_std} >> {est_std} >> {pst_std}")

outputs

2018-11-01 01:30:00 >> 2018-11-01 01:30:00-04:00 >> 2018-10-31 22:30:00-07:00
2018-11-06 01:30:00 >> 2018-11-06 01:30:00-05:00 >> 2018-11-05 22:30:00-08:00

How to create a Timezone aware Time object that works with all dates?

Answers (2)

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