Darian calendar

The Darian calendar is a proposed system of time-keeping designed to serve the needs of any possible future human settlers on the planet Mars. It was created by aerospace engineer and political scientist Thomas Gangale in 1985 and named by him after his son Darius. It was first published in June 1986.

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 (which corresponds to 686.86 days on Earth). 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, producing 6,686 sols per ten years (668.6 sols per year).

A 1998 iteration of the Darian calendar made years divisible by 100 common years, but years divisible by 500 stay leap years. However, this static intercalation scheme did not take into account the slowly increasing length of the Martian vernal equinox year. In 2006 Gangale devised a series of intercalation formulas, all of which have in common the basic decennial cycle, as shown in the following table:

This extended intercalation scheme results in an error of only about one sol at the end of 12,000 Martian years, or about the year 24,180 of the Common Era.

The year is divided into 24 months. The first 5 months in each quarter have 28 sols. The final month has only 27 sols unless it is the final month of a leap year when it contains the leap sol as its final sol.

The calendar maintains a seven-sol week, but the week is restarted from its first sol at the start of each month. If a month has 27 sols, this causes the final sol of the week to be omitted. This is partly for tidiness. It can also be rationalised as making the average length of the Martian week close to the average length of the Terrestrial week, although it must be remembered that 28 Earth days is roughly equal to  27 ¹⁄₄ Martian sols and not  27 ⁵⁄₆ Martian sols.

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