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In Bill and Ted's Excellent Adventure, the duo travel all through time. After they accidentally travel to 1,000,000 BC, they repair their booth and attempt to return to their "present" but arrive "the night before, just before they initially leave." After speaking with Rufus, they determine they need to go forward to tomorrow, to arrive at their "present." (This does not stand up in the sequel, where they go away for a time to take guitar lessons, and then return just a few seconds after they left).

Shouldn't their present be the exact moment that they left, otherwise they are traveling into a future they don't know.

There seems to be an idea in (some) science fiction that the amount of time you spend outside your present time needs to be added to your departure date, therefore if you spend 1 day in the past, you need to return 1 day later from when you left.

This can be seen in lots of time travel stories, such as in DragonLance Legends, where Caramon and Tasslehoff travel to the future, and then return to their "present" which is months after they originally left. Additionally when Hiro travels into the past and returns to a later present (one where Ando knows he has been gone for hours, days, weeks) from Heroes.

What are the reasons that time travel might employ this requirement in stories?

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    One notable exception to this rule is Back to the Future, where Marty returned not only NOT a week after he left, but actually several minutes BEFORE he left. – eidylon Nov 21 '11 at 20:02
  • They've got to get "Back!...to the Future." – Jeff Nov 24 '11 at 16:44
  • 2
    Another notable exception is Lightning by Dean Koontz, in which you always return 11 minutes after you leave, no matter how long you stay in the past or the future. (By the way, he says it's "about" 11 minutes, but I believe it's exactly 666 seconds.) – Mr Lister Aug 7 '12 at 19:50
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Warning: this answer contains links to TV Tropes.

There are many different models of time travel. They fall into three main categories:

  • Stable time loop. What happened, happened. The timeline is fixed; traveling in time isn't going to affect that. As a time traveler, you can't change the past or prevent the future, but things can still surprise you because you didn't know everything. The storytelling usually exploits this by carefully hiding what the time-traveller doesn't know yet. In this model, if you go to the past, you come back whenever you please.

    Bill and Ted's Excellent Adventure (the series) follows this model. They could have returned whenever they pleased. Why they didn't do so, is possibly (in-story) because they followed Rufus's warning that “the clock in San Dimas is always running”, which would point towards the existence of a metatime; or (out-of-story) because viewers are assumed to be familiar with a metatime model. (What's a metatime? Read on.)

  • Alternate universes. What happened, happened in an alternate timeline. You can change the past, but if you do so, you'll return to a different future. Your old future still exists, possibly with an alternate version of you.

    Alternate universes are hard to do right, and tend to work better with no time traveling involved.

  • Temporal mutability. What is today true of yesterday, may be false tomorrow. These models tend to hold up only as long as you can suspend your disbelief, if not less.

    Temporal mutability usually involves some kind of meta-time — there's the state of the timeline today, and the state of the timeline tomorrow when you've come back from messing with it. Many stories make use of meta time, linking it to the time of the reader. A classical example of meta-time put to bad use is in Back to the Future, where changing the past causes newspaper headlines to change as one character (but not the rest of the universe) watches. For a better-structured approach to metatime (one that almost makes sense physically), read Isaac Asimov's The End of Eternity. The classic among classic is “A Sound of Thunder”, Ray Bradbury's short story that popularized the butterfly effect.

    Rufus's warning in the original Bill & Ted movie makes it the trope namer for meta-time. It's not clear that Rufus's assertion that a metatime exists is reliable, however. In any case, metatime or not, Bill & Ted exhibits a consistent storyline.

Temporal mutability is often chosen because the metatime gives the reader or viewer something to hang on to. (This is not always the case.) It also breaks out of the limitation of the stable time loop, which makes it hard to create suspense. Furthermore, in cases where there is no return to the future, the paradoxical nature of temporal mutability is not apparent.

Metatime doesn't have to flow at a rate of 1:1. When metatime is the reader's time, what matters most is the causality aspect — one thing following another — and not the exact speed at which this happens. It is nonetheless convenient for metatime to happen at a known rate, both because it sets a pace for the story and because it makes for a simpler basis for the reader.

So there you have it in a nutshell: metatime is convenient for storytelling.

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An out of universe reason for the elapsed present time would be to maintain an aire of pressure in the story. This helps to hold the audience's attention. Otherwise what would keep the heroes from just living somewhere happier, ignoring the plot and losing the audience. In the example of Bill and Ted they could've just lived in the past with the princesses.

For an in universe reason, the heroes continue to age while tromping through time and as such they must return to the time that coincides with their current age or risk destroying everything they hold near and dear to their hearts. In the case of Bill and Ted if they wait too long they'll age to a point where they would have been passed when their report was due.

You cited the second movie as breaking with this tradition. While on the surface it would seem to, there is no evidence showing that after the concert Bill and Ted (and gang) didn't jump forward in time to their new present, maintaining continuity.

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    Of course one movie that does break this mold, as pointed out by @eidylon, is the original Back to the Future. Marty set his return time to a few minutes before he went back in time, hoping to alter Doc Brown's fate. In fact, by the end of the third movie only a day or so had passed in the "present" despite the fact that Marty was now at least two weeks older (and Doc Brown was several months older). – Xantec Nov 21 '11 at 20:04

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