Convenience property to retrieve the value tuple as a tuple of
        datetime objects.

def value_as_datetime(self):
        ''' Convenience property to retrieve the value tuple as a tuple of
        datetime objects.

        '''
        if self.value is None:
            return None
        v1, v2 = self.value
        if isinstance(v1, numbers.Number):
            d1 = datetime.utcfromtimestamp(v1 / 1000)
        else:
            d1 = v1
        if isinstance(v2, numbers.Number):
            d2 = datetime.utcfromtimestamp(v2 / 1000)
        else:
            d2 = v2
        return d1, d2