Now that I think about it, Isaac Asimov's "The End of Eternity" is an interesting novel with a time-traveling organization whose members are, in fact, limited to traveling only within a span encompassed by the creation of a Time Displacement Field (or some such term). So while they can travel forward and back without a problem, they cannot travel back in time to a point earlier than the Field's origin. I recall there being some interesting discussion about social engineering implemented by the organization, and I think that organization was referenced in one of the later Foundation novels (tying it in to his Robots/Empire/Foundation series, though none of that is required reading for "The End of Eternity"). It's been a while since I read it, but certainly worth revisiting...to the library!