Convert from a ClockTime relative to a given epoch.

def from_clock_time(cls, clock_time, epoch):
        """ Convert from a ClockTime relative to a given epoch.
        """
        try:
            clock_time = ClockTime(*clock_time)
        except (TypeError, ValueError):
            raise ValueError("Clock time must be a 2-tuple of (s, ns)")
        else:
            ordinal = clock_time.seconds // 86400
            return Date.from_ordinal(ordinal + epoch.date().to_ordinal())