A Generic d20 Calendar

Some well-developed campaign worlds have a calendar. Tolkien also developed alternate calendars for Earth. These are intended to be more evocative than practical, I suspect.

The problem with timekeeping and calendar reform for Earth is that nothing matches up perfectly or in an evenly divisble way. A year is slightly less than 365.25 days, a sidereal day is about four minutes shorter than a solar day, there are 86,400 seconds in a day only on average, and we can't have a simple perpetual calendar because the Abrahamic religions have religious objections to intercalary days.

For more abstract timekeeping purposes in a game, a 364-day Generic Game Year is very similar to an Earth year, and more easily divided. The Generic Game Year has a perpetual calendar in both lunar and solar months.

The Lunar Generic Game Year is divided into 13 lunar months, each with 28 days. Each month is further subdivided into four weeks, each with seven days: Moonday, Fireday, Waterday, Treeday, Goldday, Earthday, and Sunday (as in the Japanese calendar). Moonday is the first night of the full moon, which occurs on the first Moonday, Fireday, and Waterday of the month (consequently, it's easy to tell when the lycanthropes will attack). The lunar date is given by the number of the moon. For example: "the third day of the eight moon of last year". The year begins on a Moonday, the first night of the full moon following the winter solstice.

The Solar Generic Game Year is divided into four seasons (Winter, Spring, Summer, and Fall), each of which is divided into three months: Forewinter, Highwinter, Wintersaft, Forespring, Highspring, Springsaft, Foresummer, Highsummer, Summersaft, Forefall, Highfall, and Fallsaft. Each month has 30 days, except the last month in a season, which has 31 days. This extra day is of special solar significance: Wintersaft 31 is the vernal equinox; Springaft 31 is the summer solstice; Summersaft 31 is the autumnal equinox; and Fallsaft 31 (a Sunday) is the winter solstice and the end of the year. Alternatively, you could view the solar calendar as having twelve 30-day months and four intercalary days, one between each season.

This system, of course, is agnostic about the epoch for counting years, leap days, and eclipses and other astronomical events. It doesn't account for a more unusual planetary system with a double sun, extra moons, &c, but rather represents a regular Earthlike system in a simplified, abstract way.

EDIT
Note also Paizo's Time on Golarion. Who wouldn't like a 360-day calendar, at least if you could pronounce the names of the months? ;)