For example, if the audience needs to know that approximately eight years has elapsed since your last scene, then you can have one of your characters approach another and say, “Golly, Bob, I haven’t seen you for approximately eight years.” Okay, don’t do that, but it is possible to reveal that information (subtly!) through dialogue, perhaps by saying, “Golly, Bob, you haven’t changed at all in the past four congressional elections,” or, “Golly, Bob, it seems like it was just two summer Olympics ago when we last got together.” The most important points are that all exposition of this kind must begin with the words, “Golly, Bob,” and that you shouldn't just come right out and have one character tell another how much time has elapsed. Reveal this information subtly and/or through conflict.
Another way to reveal this kind of information is to show it in your description. For example, if the audience needs to know what time it is, you can do something as simple as showing a clock, though I bet you could come up with a more creative way to reveal the time. If they need to know that a year has passed, you can do one of those cool seasonal montages where you show a flower growing, then dying, then being covered by snow, and then growing again, because that sort of device has never been used in movies. </sarcasm> But seriously, you should come up with an original way to demonstrate that time has elapsed, such as maybe the protagonist’s hair is longer or shorter, or she’s now visibly pregnant, or she has a full beard, though that last one might be better for male leads. If all else fails, you can always resort to using a "SUPER:" to display some text onscreen that spells out this information to the audience.
A good rule of thumb is that you can pretty much never go wrong by writing “DAY” or “NIGHT” at the end of your slug lines. If you ever write anything else there, first confirm that it’s the kind of information that should go in a slug line, and then make sure to reveal that information to the audience in a subtle and creative way.