Year Length and Intercalation
The basic time periods from which the calendar is constructed are the Martian solar day (sometimes called a sol) and the Martian vernal equinox year, which is slightly different from the tropical year. The sol is 39 minutes 35.244 seconds longer than the Terrestrial solar day and the Martian vernal equinox year is 668.5907 sols in length. The basic intercalation formula therefore allocates six 669-sol years and four 668-sol years to each Martian decade. The former (still called leap years even though they are more common than non-leap years) are years that are either odd (not evenly divisible by 2) or else are evenly divisible by 10. The calendar error from the actual seasons is small, but the calendar is too slow by about 13 sols every 1400 Martian tropical years.
Read more about this topic: Darian Calendar
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