“Game time is of utmost importance. Failure to keep careful track of time expenditure by player characters will result in many anomalies in the game. The stricture of time is what makes recovery of hit points meaningful. Likewise, the time spent adventuring in wilderness areas removes concerned characters from their bases of operations – be they rented chambers or battlemented strongholds. Certainly the most important time strictures pertains to the manufacturing of magic items, for during the period of such activity no adventuring can be done. Time is also considered in gaining levels and learning new languages and more. All of these demands upon game time force choices upon player characters and likewise number their days of game life…YOU CAN NOT HAVE A MEANINGFUL CAMPAIGN IF STRICT TIME RECORDS ARE NOT KEPT.”

Dungeon Master’s Guide (page 37), Gary Gygax