"We in the trade call this 'Excedrin Headache Number √-3.14159..."
— Larry Niven, The Theory and Practice of Time Travel
Carl Honduras: I ARE he! ...Only I'm from the future.
Minky Steve: You know, I think it's I AM he.
Carl Honduras: It's all this time travel; it makes me very confused as to which tense we should be using.
Minky Steve: There, there. It is very tricky.
"It begins to dawn on you that everything you are about to do may prove to have been a colossal waste of time."
Harry's voice trailed off into the inadequacy of English.
Dark Helmet: What the hell am I looking at? When does this happen in the movie?!
Colonel Sandurz: Now. You're looking at now, sir. Everything that happens now is happening now.
Dark Helmet: What happened to then?
Colonel Sandurz: We passed then.
Dark Helmet: When?
Colonel Sandurz: Just now! We're at now, now.
Dark Helmet: Go back to then!
Colonel Sandurz: When?
Dark Helmet: Now.
Colonel Sandurz: Now?
Dark Helmet: Now!
Colonel Sandurz: I can't.
Dark Helmet: Why?
Colonel Sandurz: We missed it
Dark Helmet: When?
Colonel Sandurz: Just now.
Dark Helmet: When will then be now?
Colonel Sandurz: Soon!
Dark Helmet: ...how soon?
Funny, you didn't hear any commotion or gunplay. But it looks like there's already been some action in here. Or there will be. You can never take tense for granted with these goons.
The major problem is quite simply one of grammar, and the main work to consult in this matter is Dr. Dan Streetmentioner's Time Traveler's Handbook of 1001 Tense Formations. It will tell you, for instance, how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. The event will be described differently according to whether you are talking about it from the standpoint of your own natural time, from a time in the further future, or a time in the further past and is further complicated by the possibility of conducting conversations while you are actually traveling from one time to another with the intention of becoming your own mother or father.
Most readers get as far as the Future Semiconditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up; and in fact in later editions of the book all the pages beyond this point have been left blank to save on printing costs.
The Hitchhicker's Guide to the Galaxy skips lightly over this tangle of academic abstration, pausing only to note that the term "Future Perfect" has been abandoned since it was discovered not to be.
"Can we get some food? I haven't eaten since later this afternoon."
— Aaron, Primer
"The Borg magic up a temporal vortex to travel back to the past, resulting in the present being altered so that the Earth looks like Al Gore's worst nightmare. The only option is to follow them through the space-time wardrobe and stop them from assimilating Earth in the past. Which turns out to be our future, or, at least, our future right now, or... carry the one... Later."
Robert Lutece: If we could perceive time as it truly was—
Rosalind Lutece: —what reason would grammar professors have to get out of bed?