def last_modified(self):
        """
        The last modified time of the requirement's source distribution archive(s) (a number).

        The value of this property is based on the :attr:`related_archives`
        property. If no related archives are found the current time is
        reported. In the balance between not invalidating cached binary
        distributions enough and invalidating them too frequently, this
        property causes the latter to happen.
        """
        mtimes = list(map(os.path.getmtime, self.related_archives))
        return max(mtimes) if mtimes else time.time()