timezone.py

# this is stolen from datetimetester.py
from datetime import timedelta, tzinfo, datetime


def first_sunday_on_or_after(dt):
    days_to_go = 6 - dt.weekday()
    if days_to_go:
        dt += timedelta(days_to_go)
    return dt


ZERO = timedelta(0)
MINUTE = timedelta(minutes=1)
HOUR = timedelta(hours=1)
DAY = timedelta(days=1)
# In the US, DST starts at 2am (standard time) on the first Sunday in April.
DSTSTART = datetime(1, 4, 1, 2)
# and ends at 2am (DST time; 1am standard time) on the last Sunday of Oct,
# which is the first Sunday on or after Oct 25.  Because we view 1:MM as
# being standard time on that day, there is no spelling in local time of
# the last hour of DST (that's 1:MM DST, but 1:MM is taken as standard time).
DSTEND = datetime(1, 10, 25, 1)


class USTimeZone(tzinfo):

    def __init__(self, hours, reprname, stdname, dstname):
        self.stdoffset = timedelta(hours=hours)
        self.reprname = reprname
        self.stdname = stdname
        self.dstname = dstname

    def __repr__(self):
        return self.reprname

    def tzname(self, dt):
        if self.dst(dt):
            return self.dstname
        else:
            return self.stdname

    def utcoffset(self, dt):
        return self.stdoffset + self.dst(dt)

    def dst(self, dt):
        if dt is None or dt.tzinfo is None:
            # An exception instead may be sensible here, in one or more of
            # the cases.
            return ZERO
        assert dt.tzinfo is self

        # Find first Sunday in April.
        start = first_sunday_on_or_after(DSTSTART.replace(year=dt.year))
        assert start.weekday() == 6 and start.month == 4 and start.day <= 7

        # Find last Sunday in October.
        end = first_sunday_on_or_after(DSTEND.replace(year=dt.year))
        assert end.weekday() == 6 and end.month == 10 and end.day >= 25

        # Can't compare naive to aware objects, so strip the timezone from
        # dt first.
        if start <= dt.replace(tzinfo=None) < end:
            return HOUR
        else:
            return ZERO


Pacific = USTimeZone(-8, "Pacific", "PST", "PDT")