def invalid_timezone(prop):
    if hasattr(prop.dt, 'tzinfo') and prop.dt.tzinfo is None and 'TZID' in prop.params:
        return True
    else:
        return False
check if a icalendar property has a timezone attached we don't understand
dataset/ETHPy150Open pimutils/khal/khal/khalendar/aux.py/invalid_timezone