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The Darian calendar is a complete timekeeping system for the 24-hour, 39-minute, 35.244-second sol and the 668.5907-sol vernal equinox year on Mars. Features include: 24 months, normally containing 28 sols, with three to four 27-sol months spaced regularly throughout the year to total either 668 or 669 sols. This results in a month whose length is within 6 percent of the mean Gregorian calendar month, within 3 percent of the lunar month, and near the statistical mean of the human menstrual cycle. A nominal seven-sol week, with six-sol weeks ending the 27-sol months, thus allowing every month to begin on the first sol of the week. The numerical sol of any month always occurs on the same sol of the week. Since the new year always begins on the first sol of the week, there are only two types of calendar years: one common year and one bissextile or “leap” year.