It's a good idea. Why? Well, lets examine the usual "you travel back in time, and need to convince your (past) self to help your (future) self" scenario.

(Future) Self calls (Past) Self:

PSelf: Hello?
FSelf: Hi! This is you, from twenty years in the future! Do not press the big, red..
PSelf: Erm, WHO is this? My future self? Right... HA HA HA *Click*

Obviously, this is no way for FSelf to go about convincing PSelf to help FSelf. Granted, FSelf might be able to get PSelf to believe FSelf eventually -- FSelf does know all the dark secrets of PSelf's life, but this could simply serve to convince PSelf that FSelf is a voyeuristic stalker.

If FSelf met PSelf in person, things would be easier, but is PSelf really going to believe the whole "traveled back in time" thing when a much simpler "evil twin" scenario is so obvious? If so, maybe PSelf should visit Occam's Razor. And what if FSelf needs to send someone in their place to convince PSelf to do something?

Now, we address the problem of this ever actually happening. Granted, the chances are very slim that you'll be hurled back in time and need to convince yourself to do things, but you must admit that such a possibility exists. Even if you are steadfastly convinced that it doesn't, this is an interesting exercise in temporal theoretics, so stick with me.

Anyway, here's what you do. Think up a secret phrase, or, shall we say, "password". Don't make it something common. "chair" is a stupid temporal password. If you pick "chair", FSelf's world DESERVES to be destroyed.

So, you have a reasonably intricate password.
  • DO NOT tell anyone.
  • DO NOT write it down.
  • DO NOT forget it.

    Now, we come back to the phone conversation.

    PSelf: Hello?
    FSelf: This is your future self. The password is "argle the chemical puppy". It is vitally important to the safety of the universe that you not press the red button.
    PSelf: Well, gee, guess I won't then. Anything else I can do?

    Clearly, this is a far more effective way to save the world.