Mars Leap Year System
On Earth, our calendar is based on 365 days, but the mean solar year is 365.24219 days long. In order to be synchronous with the seasons (i.e., such that the equinox and solstice dates do not vary from year to year), our calendar mandates a leap year every 97 out of 400 years. This means an extra day is inserted on February 29th during leap years. Thus, the average year is lengthened to 365.2425 days, and Earth's calendar stays aligned with the seasonal variations, except for the additional fraction of 0.00031 days, which will cause a one day error in about 3226 years.
We would like the Martian Calendar to be synchronous with the seasons on Mars also. But since the Martian mean solar year is 668.599 sols long, and our Martian calendar is based on a 668 sol year, the seasons will lag behind the calendar dates by .599 sols per year. A simple leap year scheme that would be accurate to about 1 day in 1000 years for Mars would be to have a leap year for any year divisible by two or five. This system is illustrated below. The cycle would repeat every 10 years. This system yields a 669.6 sol Martian year on average. The extra leap sol could be arbitrarily inserted at the end of February (51st) similar to Earth's leap day.
Copyright 1996-98 F. N. Bauregger