Time and Movement (PHB)

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As in the real world, time passes in all AD&D game worlds. Weeks slip away as wizards research spells. Days go by as characters ride across country. Hours pass while exploring ruins. Minutes flash by during battles. All of these are passages of time.
There are two different types of time that are talked about in these rules. Game time is the imaginary time that passes for the characters in the game. Real time is the time in the real world, the time that passes for the players and DM as they play the AD&D game. The two times are very different; players and DMs should be careful to distinguish game time from real time.
For example, when the character Delsenora researches a spell for three weeks, this is three weeks of game time. Delsenora is out of action as three weeks pass in the campaign world. Since nothing interesting at all happens to Delsenora during this research time, it should require only a minute or two of real time to handle the situation. The exchange in real time is something like this:
: Louise (Delsenora's player): "Delsenora's going to research her new spell."
: DM: "OK, it'll take three weeks. Nothing happens to her. While she's doing that, the rest of you get a chance to heal your wounds and do some stuff that you've been ignoring. Johann [pointing at another player], you'd better spend some time at the church. The patriarch's been a little upset that you haven't been attending ceremonies."
: Jon (Johann's player): "Can't I go out and earn some more experience?"
: DM (Not wanting to deal with a split-up group): "The patriarch mumbles something about failing in your duties to your deity, and he rubs his holy symbol a lot. You know, it's not very often that low levels like you have personal audiences with the patriarch. What do you think?"
: Jon: "Marvelous. Subtle hint. I'll stay and be a good boy."
: DM: "Well, great! The three weeks pass. Nothing happens. Del, make your roll for the spell research."
And so three weeks of game time flash by in brief minutes of real time.
The importance of game time is that as a campaign progresses, characters tend to become involved in different time-consuming projects. Three characters may set off on a four-week overland journey, while a wizard researches for six weeks. At the inn, a fighter rests and heals his wounds for two weeks. It is important to note how much time passes during different tasks, so the activities of different characters can be followed.
Campaign time is measured just as it is in real life: years, months, weeks, days, hours, minutes, and seconds. But, since this is a fantasy game, the DM can create entirely different calendars for his world. There may be only 10 months in the year or 63 days to a month. When beginning play, these things are not tremendously important, so players need not worry about them right away. With continued adventuring, players eventually become familiar with the calendar of the campaign.
Rounds and turns are units of time that are often used in the AD&D game, particularly for spells and combat. A round is approximately equal to a minute (it is not exactly one minute, so as to grant the DM some flexibility during combat). A turn is equal to 10 minutes of game time. Turns are normally used to measure specific tasks (such as searching) and certain spells. Thus, a spell that lasts 10 turns is equal to 100 minutes or one and two-thirds hours.