@classmethod
def to_seconds(cls, timing, unit, model):
    """converts the schedule values to seconds, depending on to the
        schedule_model the value will differ. So if the schedule_model is
        'effort' or 'length' then the schedule_time and schedule_unit values
        are interpreted as work time, if the schedule_model is 'duration' then
        the schedule_time and schedule_unit values are considered as calendar
        time.
        """
    if not unit:
        return None
    lut = {'min': 60, 'h': 3600, 'd': 86400, 'w': 604800, 'm': 2419200, 'y':
        31536000}
    if model in ['effort', 'length']:
        from stalker import defaults
        day_wt = defaults.daily_working_hours * 3600
        week_wt = defaults.weekly_working_days * day_wt
        month_wt = 4 * week_wt
        year_wt = int(defaults.yearly_working_days) * day_wt
        lut = {'min': 60, 'h': 3600, 'd': day_wt, 'w': week_wt, 'm':
            month_wt, 'y': year_wt}
    return timing * lut[unit]
