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 mean-ingful. Likewise, the time spent adventuring in wilderness areas removes concerned characters from their bases of operation - be they rented chambers or battlemented strongholds. Certainly the most important time stricture pertains to the manufacture 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.
One of the things stressed in the original game of D&D was the importance of recording game time with respect to each and every player character in a campaign. In AD&D it is emphasized even more:
YOU CAN NOT HAVE A MEANINGFUL CAMPAIGN IF STRICT TIME RECORDS ARE NOT KEPT.
Use whatever grouping of days you find desirable for your milieu. There is nothing wrong with 7 day weeks and 31, 30 and 28/29 day months which exactly correspond to our real system. On the other hand, there is nothing to prevent you from using some other system if it pleases you and you can keep it straight. What is important to the campaign is that you do, in fact, maintain a time record which logs the activities and whereabouts of player characters and their henchmen.
For the sake of example, let us assume that you begin your campaign on Day 1 of the Year 1000. There are four player characters who begin ini-tially, and they have adventures which last a total of 50 days — 6 days of actual adventuring and 44 days of resting and other activity.
At this point in time two new players join the game, one of the original group decides to go to seek the advice of an oracle after hiring an elven henchman, and the remaining three "old boys" decide they will not go with the newcomers. So on Day 51 player A's character is off on a journey, those of B, C, and D are resting on their laurels, and E and F enter the dungeon. The latter pair spend the better part of the day surviving, but do well enough to rest a couple of game days and return for another try on Day 54 — where they stumble upon the worst monster on the first level, surprise it, and manage to slay it and come out with a handsome treasure. You pack it in for the night.
Four actual days later (and it is best to use 1 actual day = 1 game day when no play is happening), on Day 55, player characters B, C, and D enter the dungeon and find that the area they selected has already been cleaned out by player characters E and F. Had they come the day after the previous game session, game Day 52, and done the same thing, they would have found the monster and possibly gotten the goodies!
What to do about that? and what about old A and his pointy-eared chum off to see the oracle?