def set_timestep(self, timestep=const.seconds_per_day, num_steps_per_year=None):
if num_steps_per_year is not None:<EOL><INDENT>timestep = const.seconds_per_year / num_steps_per_year<EOL><DEDENT>self.timestep = timestep<EOL>
Calculates the timestep in unit seconds
        and calls the setter function of :func:`timestep`

        :param float timestep:              the amount of time over which
                                            :func:`step_forward` is integrating
                                            in unit seconds [default: 24*60*60]
        :param float num_steps_per_year:    a number of steps per calendar year
                                            (optional)

        If the parameter *num_steps_per_year* is specified and not ``None``,
        the timestep is calculated accordingly and therefore the given input
        parameter *timestep* is ignored.