@classmethod
def lookup_relativedelta_parameter(cls, unit_string):
    """
        >>> lrp = Schedule.lookup_relativedelta_parameter
        >>> lrp('Years')
        'years'
        >>> lrp('yr')
        'years'
        >>> lrp('s')
        'seconds'
        """
    try:
        return cls._rdp_map()[unit_string.lower()]
    except KeyError:
        raise ValueError("%s doesn't match any supported time/date unit")
