Babylon 5 - whose major plot device was the disappearance of B4 for use in a war 1000 years prior, which reduced the forces of the enemy they were fighting in the 'now', which was engineered by the first commander of B5 who went back with B4 and became a prophet to warn them about the next war. This included a subplot in which the second commander of B5 teams up with the first (and others) to jump back and prevent the destruction of B4 before they can move it back through time - which would of course alter 'history' that's already happened...
Douglas Adams had a whole section on grammatical tenses for use in such circumstances, but I only got as far as the Future Semiconditionally Modified Subinverted Plagal Past Subjunctive Intentional before I gave up.
