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tzinfo_examples.py
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from datetime import tzinfo, timedelta, datetime ZERO = timedelta(0) HOUR = timedelta(hours=1) SECOND = timedelta(seconds=1) # A class capturing the platform's idea of local time. # (May result in wrong values on historical times in # timezones where UTC offset and/or the DST rules had # changed in the past.) import time as _time STDOFFSET = timedelta(seconds = -_time.timezone) if _time.daylight: DSTOFFSET = timedelta(seconds = -_time.altzone) else: DSTOFFSET = STDOFFSET DSTDIFF = DSTOFFSET - STDOFFSET class LocalTimezone(tzinfo): def fromutc(self, dt): assert dt.tzinfo is self stamp = (dt - datetime(1970, 1, 1, tzinfo=self)) // SECOND args = _time.localtime(stamp)[:6] dst_diff = DSTDIFF // SECOND # Detect fold fold = (args == _time.localtime(stamp - dst_diff)) return datetime(*args, microsecond=dt.microsecond, tzinfo=self, fold=fold) def utcoffset(self, dt): if self._isdst(dt): return DSTOFFSET else: return STDOFFSET def dst(self, dt): if self._isdst(dt): return DSTDIFF else: return ZERO def tzname(self, dt): return _time.tzname[self._isdst(dt)] def _isdst(self, dt): tt = (dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second, dt.weekday(), 0, 0) stamp = _time.mktime(tt) tt = _time.localtime(stamp) return tt.tm_isdst > 0 Local = LocalTimezone() # A complete implementation of current DST rules for major US time zones. def first_sunday_on_or_after(dt): days_to_go = 6 - dt.weekday() if days_to_go: dt += timedelta(days_to_go) return dt # US DST Rules # # This is a simplified (i.e., wrong for a few cases) set of rules for US # DST start and end times. For a complete and up-to-date set of DST rules # and timezone definitions, visit the Olson Database (or try pytz): # http://www.twinsun.com/tz/tz-link.htm # http://sourceforge.net/projects/pytz/ (might not be up-to-date) # # In the US, since 2007, DST starts at 2am (standard time) on the second # Sunday in March, which is the first Sunday on or after Mar 8. DSTSTART_2007 = datetime(1, 3, 8, 2) # and ends at 2am (DST time) on the first Sunday of Nov. DSTEND_2007 = datetime(1, 11, 1, 2) # From 1987 to 2006, DST used to start at 2am (standard time) on the first # Sunday in April and to end at 2am (DST time) on the last # Sunday of October, which is the first Sunday on or after Oct 25. DSTSTART_1987_2006 = datetime(1, 4, 1, 2) DSTEND_1987_2006 = datetime(1, 10, 25, 2) # From 1967 to 1986, DST used to start at 2am (standard time) on the last # Sunday in April (the one on or after April 24) and to end at 2am (DST time) # on the last Sunday of October, which is the first Sunday # on or after Oct 25. DSTSTART_1967_1986 = datetime(1, 4, 24, 2) DSTEND_1967_1986 = DSTEND_1987_2006 def us_dst_range(year): # Find start and end times for US DST. For years before 1967, return # start = end for no DST. if 2006 < year: dststart, dstend = DSTSTART_2007, DSTEND_2007 elif 1986 < year < 2007: dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006 elif 1966 < year < 1987: dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986 else: return (datetime(year, 1, 1), ) * 2 start = first_sunday_on_or_after(dststart.replace(year=year)) end = first_sunday_on_or_after(dstend.replace(year=year)) return start, end 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 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 start, end = us_dst_range(dt.year) # Can't compare naive to aware objects, so strip the timezone from # dt first. dt = dt.replace(tzinfo=None) if start + HOUR <= dt < end - HOUR: # DST is in effect. return HOUR if end - HOUR <= dt < end: # Fold (an ambiguous hour): use dt.fold to disambiguate. return ZERO if dt.fold else HOUR if start <= dt < start + HOUR: # Gap (a non-existent hour): reverse the fold rule. return HOUR if dt.fold else ZERO # DST is off. return ZERO def fromutc(self, dt): assert dt.tzinfo is self start, end = us_dst_range(dt.year) start = start.replace(tzinfo=self) end = end.replace(tzinfo=self) std_time = dt + self.stdoffset dst_time = std_time + HOUR if end <= dst_time < end + HOUR: # Repeated hour return std_time.replace(fold=1) if std_time < start or dst_time >= end: # Standard time return std_time if start <= std_time < end - HOUR: # Daylight saving time return dst_time Eastern = USTimeZone(-5, "Eastern", "EST", "EDT") Central = USTimeZone(-6, "Central", "CST", "CDT") Mountain = USTimeZone(-7, "Mountain", "MST", "MDT") Pacific = USTimeZone(-8, "Pacific", "PST", "PDT")