def advance_by(self, amount):
        """Advance the time reference by the given amount.

        :param `float` amount: number of seconds to advance.
        :raise `ValueError`: if *amount* is negative.
        """
        if amount < 0:
            raise ValueError("cannot retreat time reference: amount {} < 0"
                             .format(amount))
        self.__delta += amount
Advance the time reference by the given amount.

        :param `float` amount: number of seconds to advance.
        :raise `ValueError`: if *amount* is negative.