Collision with Chronos

The ruins were getting younger. They were thought to be the ruins of an invading force of space monsters that men had defeated during the Dark Ages centuries before. But the ruins were visibly getting newer—rebuilding themselves. The militarists who had reconstructed society after the supposed invasion were getting panicky. Until they found a complete invader vehicle—and learned it travelled through time. But what was Time? What was Now? Could there be more than one Time Front—one going forward, one in reverse? And what would happen when two such fronts met in the inevitable Collision Course?