Differences in the length of days and years makes the Earth calendar useless on Mars. To remedy this, Martians have developed their own system of timekeeping which, though it roughly corresponds to the Earth system, uses different terms and methods of calculation.
Martian “days” are called sols, a term in use by astronomers since the 20th century. One sol is 24 hours and 39 minutes long in Earth time. Martian sols are divided into an Earth-like system of 24 hours with sixty minutes of sixty seconds each. As sols are longer than days, Martian seconds, minutes and hours are slightly longer than those on Earth.
Sols are group into seven day weeks, in keeping with Terran tradition. Martian sols are called, in order, Heliosol, Phobosol, Deimosol, Terrasol, Jovisol, Venusol, and Saturnisol.
A Martian year is 668.5907 sols in length. Mars uses a 24 month calendar which is broken into quarters with each month having 28 sols except for the last month of the quarter, which contains only 27 sols, for a total of 668 sols. Odd numbered years and years evenly divisible by ten, called leap years, contain an extra sol which is added at the end, giving the last month of the last quarter 28 sols, for a total of 669 sols.
The extra day of the ten year cycle is called the Decennial and is not numbered or considered as a day of the week. It is essentially an extra day, marked as a celebration sol.
The Martian year begins on the northern hemisphere’s vernal equinox. Months are named after the sign of the zodiac which is roughly dominant during its timeframe. As there are only 12 zodiac signs, two consecutive months share the same name and are designated as “First” or “Second,” written as simply “1″ or “2.”
The Martian year is 1.88 times the length of the Earth year. Year 0 in the Martian system begins in Earth year 1609, the beginning of the Telescopic Era and humankind’s first close glimpse at the planet.
The Cwynhild’s Loom episode “The Space Between the Lines” takes place on Venusol 2 Libra 17, 400.