timezone.py

from datetime import timedelta, 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

# For a complete and up-to-date set of DST rules and timezone definitions,
# visit the Olson Database: http://www.twinsun.com/tz/tz-link.htm or
# http://sourceforge.net/projects/pytz/

# 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

def dst(dt, dst):
    if not dst:
        return dt
    # Find start and end times for US DST. For years before 1967, return dt
    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 dt 

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

    if start <= dt < end:
        return timedelta(hours=1) + dt
    else:
        return dt