def get_iana_tz(windows_tz):
    """ Returns a valid pytz TimeZone (Iana/Olson Timezones) from a given
    windows TimeZone

    :param windows_tz: windows format timezone usually returned by
     microsoft api response
    :return:
    :rtype:
    """
    timezone = WIN_TO_IANA.get(windows_tz)
    if timezone is None:
        # Nope, that didn't work. Try adding "Standard Time",
        # it seems to work a lot of times:
        timezone = WIN_TO_IANA.get(windows_tz + ' Standard Time')

    # Return what we have.
    if timezone is None:
        raise pytz.UnknownTimeZoneError(
            "Can't find Windows TimeZone " + windows_tz)

    return timezone