The calendar most of us use today is called the Gregorian calender, after Pope Gregory XIII. He adjusted the Julian calendar (devised by Julius Caeser) because the drift of days was causing issues with the celebration of Easter.
Easter is a wacky thing. It's celebrated on the first Sunday after Passover, because the Last Supper (the meal reenacted by the Eucharist, the drinking of wine and the eating of bread during Mass) was the traditional Passover meal. This is why communion host tends to be flat; it was the same unleavened bread that Jews traditionally have with the Passover meal.
Anyway, the celebration of Passover is dictated by a lunar calendar, while the Julian calender is solar. The two don't sync up exactly, and because a day isn't exactly 24 hours long, things were getting kinda wacky by the time Gregory was pope. So he recreated the calender and whacked 10 days out of it (specifically Friday, October 5th to Thursday, October 14th of 1582). Adjustments to which years are leap years made the solar Gregorian calender match up a little more closely with reality.
It's possible to invest the time and energy to make your gaming calendars that complex as well, and you can get some really neat effects with multiple moons or even multiple suns. However, it's generally not a good idea to mess with days and weeks. People are used to 24 hour days and 7 day weeks, and if you muck with that, people tend to forget. It's generally more trouble than its worth.
After that, you can go crazy if you like. Months and years tend to be longer than most people need to deal with regularly. Beyond knowing how much food is in a weekly packet of iron rations and how often daily powers rejuvenate, time generally isn't terribly interesting to adventurers. Most tend to be willing to take your word for it when you tell them when the next full moon is, or the summer solstice.
Since my Labyrinth Lord campaign is an early Iron Age setting, a simpler calendar would probably work best to reinforce that. on the other hand, the dominant empires of the past were run by lizard folk, and I've slightly modeled aspects of their culture after the Mayans who had some really funky calendars.
First, I'm going to start with the basics: 24 hour days, 7 day weeks. If I say that 4 weeks (28 days) makes up the time it takes the moon to go through a complete cycle, and that there are three such cycles per season, and four seasons per year, that gives me 1 year = 4 seasons = 12 Moons = 336 days. That's noticeably shorter than our 365 day year, but I doubt anyone is going to get seriously bent out of shape over it.
Each full moon begins the Moon of 28 days (and we're using the silver moon here, of course, since the red moon is fairly new). Because this world is a tad more finely tuned than our own, the solstices and equinoxes also fall on these full moons, and different cultures have various ways of marking these occasions. The year begins and ends with the winter solstice, so the winter solstice is the first day of Winter as well as being the first day of the new year.
But which year? It's fun to come up with crazy names for your years, the Year of the Cranky Ostrich, the Year of the Inebriated Octopus, etc. My game, however, deals with a lot of history, and it's just simpler on everyone if I used numbered years. Most numbered calendars use a significant event to start the count, a sort of year zero. Right now in my campaign, most use a version of the original lizard folk calendar, but year 0 is generally agreed to be the founding of the Second Lizard Folk Empire, 612 years ago. So most describe the current year as the 612th Year of the Second Empire. Others might use other counts, such as the elves for whom it is the 703rd Year Since the Scouring, when Tiamat destroyed their empire.
Like the Mayans, however, the Lizard Folk have a Long Count as well, stretching back to the first day of Creation, and forward to the year 987,654. On the first day of the second week of the third Moon of that year will happen an event the sages of the First Empire called the Great Conjunction. Nobody knows exactly what they meant by that, but since the current year is only 126,984 in the Long Count nobody is exactly losing any sleep over it.
The current date is the 4th day of the second week of the second Moon of Autumn of the 612th year of the Second Empire, 703rd year Since the Scouring, and 126,984th year of Earth and Ocean's imprisonment.
(Woohoo! I figured out how to make it so if you click on the photos in this post, you get imbigified versions. This way you can get a much better feel for the artistry of my great photographers!)
Photo credits: timatymusic, *L*u*z*a* return to nature
, mike 23.