A Martian year. This term is used by writer Kim Stanley Robinson in his series of Mars novels to refer to the length of time it takes for Mars to travel around the sun. One m-year is 669 Martian days long, a Martian day being 24 hours and 31 minutes long. This is dealt with in an interesting and innovative way; at midnight, all clocks stop for 30 minutes. The Martian timeslip, the half-hour period between 00:00 and 00:01 is when Martians party the hardest.
The Martian year is divided up into 24 months (January 1, January 2, Februrary 1, etc.) which are 28 days long, except for three, which are 27 days long.
One final quirk of Martian timekeeping: the seasons are not all of the same length. Because of the eccentricity of Mars' orbit, in the Northern hemisphere spring is 194 days long, summer is 178 days long, autumn is 143 days long and winter is 154 days long.
Information taken from The Martians by Kim Stanley Robinson.