tz_tools.py

from tzwhere import tzwhere
from datetime import datetime
import pytz

# timezone lookup, force nearest tz for coords outside of polygons
WHERETZ = tzwhere.tzwhere(shapely=True, forceTZ=True)
# daylight savings time (DST) in northern hemisphere starts in March and ends
# in November and the opposite in southern hemisphere
JAN1 = datetime(2016, 1, 1)  # date with standard time in northern hemisphere
JUN1 = datetime(2016, 6, 1)  # date with standard time in southern hemisphere

# notes and links on Python datetime tzinfo dst:
# http://stackoverflow.com/questions/17173298/is-a-specific-timezone-using-dst-right-now
# http://stackoverflow.com/questions/19774709/use-python-to-find-out-if-a-timezone-currently-in-daylight-savings-time

# methods for determining DST vs. STD
# tz.localize(JAN1).timetuple().tm_isdst -> boolean, True if DST, False if STD
# tz.localize(JAN1).dst() -> timedelta, 0 if STD
# tz.dst(JAN1) -> timedelta, 0 if STD


def tz_latlon(lat, lon):
    """
    Timezone from latitude and longitude.

    :param lat: latitude [deg]
    :type lat: float
    :param lon: longitude [deg]
    :type lon: float
    :return: timezone
    :rtype: float
    """

    # get name of time zone using tzwhere, force to nearest tz
    tz_name = WHERETZ.tzNameAt(lat, lon, forceTZ=True)

    # check if coordinates are over international waters
    if not tz_name or tz_name in ('uninhabited', 'unknown'):
        # coordinates over international waters only depend on longitude
        return lon // 15.0
    else:
        tz_info = pytz.timezone(tz_name)  # get tzinfo

    # get the daylight savings time timedelta
    tz_date = JAN1  # standard time in northern hemisphere
    if tz_info.dst(tz_date):
        # if DST timedelta is not zero, then it must be southern hemisphere
        tz_date = JUN1  # a standard time in southern hemisphere
    tz_str = tz_info.localize(tz_date).strftime('%z')  # output timezone from ISO
    # convert ISO timezone string to float, including partial timezones
    return float(tz_str[:3]) + float(tz_str[3:]) / 60.0, tz_info


if __name__ == "__main__":
    # test tz_latlon at San Francisco (GMT+8.0)
    gmt, tz_info = tz_latlon(37.7, -122.4)
    assert gmt == -8.0
    assert tz_info.zone == 'America/Los_Angeles'
    assert tz_info.utcoffset(JAN1).total_seconds()/3600.0 == -8.0
    assert tz_info.utcoffset(JUN1).total_seconds()/3600.0 == -7.0
    # New_Delhi, India (GMT-5.5)
    gmt, tz_info = tz_latlon(28.6, 77.1)
    assert gmt == 5.5
    assert tz_info.zone == 'Asia/Kolkata'
    assert tz_info.utcoffset(JAN1).total_seconds()/3600.0 == 5.5
    assert tz_info.utcoffset(JUN1).total_seconds()/3600.0 == 5.5
    # also works with integers (EG: Santa Cruz, CA)
    gmt, tz_info = tz_latlon(37, -122)
    assert gmt == -8.0
    assert tz_info.zone == 'America/Los_Angeles'