Keeping a campaign calendar is always a drag. I don't know about you but I always feel that it needs to be done, but its obnoxious as heck to track everything. Especially when the party goes into "dungeon time," i.e., the treadmill of sleeping for 16 hours to rest and prep spells and then kicking in a door and calling it a (15 minute) day.
Plus, when you come up with your own campaign calendar (I've used the Greyhawk one, myself) then it always has problems, namely that nobody really knows exactly what each month is called, or what the names are etc.
The quick off the cuff idea: Reduce the granularity!
At the end of each session/quest/break point, think about how many days have passed (roughly, if necessary), or how many times the party has slept to rest. Then, roll 1d6. If the number on the die is less than the number of days you think have passed, mark off a week.
Sometimes a whole week will pass in what felt like a day. A feeling of time distortion is one of the dangers in exploring strange dungeons.
I think it'd be much easier to just mark off weeks as the basic units of time for tracking operational campaigns rather than tracking each and every day.
OD&D Experience Levels
8 hours ago