'Check whether the field type is date/time (lazy property)
@return: True if field type is datetime, date or time, else False'

@property
def is_datetime(self):

    is_datetime = self._is_datetime
    if (is_datetime is None):
        is_datetime = (self.ftype in ('datetime', 'date', 'time'))
        self._is_datetime = is_datetime
    return is_datetime
