from datetime import tzinfo, timedelta, datetime

ZERO = timedelta(0)

HOUR = timedelta(hours=1)

# A UTC class.

class UTC(tzinfo):

    """UTC"""

    def utcoffset(self, dt):

        return ZERO

    def tzname(self, dt):

        return "UTC"

    def dst(self, dt):

        return ZERO

utc = UTC()

# A class building tzinfo objects for fixed-offset time zones.

# Note that FixedOffset(0, "UTC") is a different way to build a

# UTC tzinfo object.

class FixedOffset(tzinfo):

    """Fixed offset in minutes east from UTC."""

    def __init__(self, offset, name):

        self.__offset = timedelta(minutes = offset)

        self.__name = name

    def utcoffset(self, dt):

        return self.__offset

    def tzname(self, dt):

        return self.__name

    def dst(self, dt):

        return ZERO

# A class capturing the platform's idea of local time.

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 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, -1)

        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; 1am standard time) on the first Sunday of Nov.

DSTEND_2007 = datetime(1, 11, 1, 1)

# 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; 1am standard 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, 1)

# 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;

# 1am standard 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

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

        # Find start and end times for US DST. For years before 1967, return

        # ZERO for no DST.

        if 2006 < dt.year:

            dststart, dstend = DSTSTART_2007, DSTEND_2007

        elif 1986 < dt.year < 2007:

            dststart, dstend = DSTSTART_1987_2006, DSTEND_1987_2006

        elif 1966 < dt.year < 1987:

            dststart, dstend = DSTSTART_1967_1986, DSTEND_1967_1986

        else:

            return ZERO

        start = first_sunday_on_or_after(dststart.replace(year=dt.year))

        end = first_sunday_on_or_after(dstend.replace(year=dt.year))

        # 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

Eastern  = USTimeZone(-5, "Eastern",  "EST", "EDT")

Central  = USTimeZone(-6, "Central",  "CST", "CDT")

Mountain = USTimeZone(-7, "Mountain", "MST", "MDT")

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