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?