Showing From Time to Time by Jack Finney

Ruben Prien is still at work with the Project, still dreaming of altering man's fate by going back in time to adjust events... to interfere, some might say, with destiny. Once again, his conduit to that bygone era, his messenger to that lost world, is Simon Morley, the man who actually proved himself capable of traveling back and forth in time.
Rube's purpose in summoning Si back from that earlier world, where he has taken up permanent residence, is no less grand than an attempt to prevent World War I from erupting. It is ironic, therefore, that the man assigned to carry to America the papers that might help avert the Great Catastrophe travels to his meeting on board the Titanic. And it is Si's task to attempt to ensure his safe passage.