Advance the time reference so that now is the given timestamp.

        :param `float` timestamp: the new current timestamp.
        :raise `ValueError`: if *timestamp* is in the past.

def advance_to(self, timestamp):
        """Advance the time reference so that now is the given timestamp.

        :param `float` timestamp: the new current timestamp.
        :raise `ValueError`: if *timestamp* is in the past.
        """
        now = self.__original_time()
        if timestamp < now:
            raise ValueError("cannot retreat time reference: "
                             "target {} < now {}"
                             .format(timestamp, now))
        self.__delta = timestamp - now