The length of a year is determined by the earth's orbit around the sun. Sure, you can make a calendar that isn't actually a year long, and then correct the errors that accumulate every once in a while, either by inserting a few months every few hundred years, or by adding a leap day every 4 years, but why would you intentionally make a calendar that isn't a year long? Leap days are unavoidable, but seasons drifting out-of-sync are not.