@staticmethod
def date_diff(time_point_1=None, time_point_2=None):
    """Return (duration, is_negative) between two TimePoint objects.

        duration -- is a Duration instance.
        is_negative -- is "-" if time_point_2 is in the past of time_point_1.
        """
    if time_point_2 < time_point_1:
        return time_point_1 - time_point_2, '-'
    else:
        return time_point_2 - time_point_1, ''
