Talk:Moons/@comment-24766702-20140831193659/@comment-10390252-20140907203438

The lunar month (new moon to new moon) is about 28 days, which doesn't go precisely into the solar year (winter solstice to winter solstice) of 365 days. That's why most ancient calendars, instead of having a leap-year with just one month one day longer, have a thirteenth month every four or five years.

These time measurements go back at least 4000 to 5000 years and seem universal to every culture on Earth.

That said, remember that the moon moves under intelligent control in Equestria, so there is no telling precisely how long it's cycle is unless Luna (and later Celestia) regulated it to a certain number of days to assist in time-keeping.