@abc.abstractmethod<EOL><INDENT>def get_num_hours(self):<DEDENT>
return<EOL>
Gets the number of hours in a day.

        For a variable hour time system, the number of hours defined is
        returned. If there are no "hours" in this time system then this
        value may be zero.

        :return: the number of hours
        :rtype: ``cardinal``


        *compliance: mandatory -- This method must be implemented.*