Returns a tuple of (earliest, latest) dates on which the service periods
    in the schedule define service, in YYYYMMDD form.

def GetDateRange(self):
    """Returns a tuple of (earliest, latest) dates on which the service periods
    in the schedule define service, in YYYYMMDD form.
    """
    (minvalue, maxvalue, minorigin, maxorigin) = self.GetDateRangeWithOrigins()
    return (minvalue, maxvalue)