The Darian calendar is a proposed system of timekeeping designed to serve the needs of any possible future human settlers on the planet Mars. It was created by aerospace engineer, political scientist, and space jurist Thomas Gangale in 1985 and named by him after his son Darius. It was first published in June 1986.Gangale, Thomas. (1986-06-01). "Martian Standard Time". Journal of the British Interplanetary Society. Vol. 39, No. 6, p. 282–288. In 1998 at the founding convention of the Mars Society the calendar was presented as one of two calendar options to be considered along with eighteen other factors to consider for the colonization of Mars.
Due to the use of 28 sol months, the Darian calendar has no mechanism for synchronization with Earth dates or its synodic periods. It is specifically engineered for Martian cycles, whereas the Gregorian calendar is built for Earth's orbital period and day length. For future settlers, the two systems serve different roles: one provides local seasonal accuracy on Mars, while the other maintains a connection to Earth's legal and social standards.
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. 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 686.9711 days on Earth).
The basic intercalation formula therefore allocates six 669-sol years and four 668-sol years to each Martian decade. The former are still called leap years, even though they are more common than non-leap years, and are years that are either odd (not evenly divisible by 2) or are evenly divisible by 10: this produces 6,686 sols per ten years, giving an average year of 668.6 sols.
A 1998 iteration of the Darian calendar had leap years cancelled if the year was divisible by 100, unless the year was also divisible by 500; adding these rules produces an average year of 668.592 sols, a more reasonable approximation.Gangale, Thomas. (1998-08-01). "The Darian Calendar". Mars Society. MAR 98-095. Proceedings of the Founding Convention of the Mars Society. Volume III. Ed. Robert M. Zubrin, Maggie Zubrin. San Diego, California. Univelt, Incorporated. 13-Aug-1998.
However, these static intercalation schemes did not take into account the slowly increasing length of the Martian vernal equinox year. Thus, 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:
Range of years Formula Mean length of calendar year 0–2000 (Y − 1)\2 + Y\10 − Y\100 + Y\1000 668.5910 sols 2001–4800 (Y − 1)\2 + Y\10 − Y\150 668.5933 sols 4801–6800 (Y − 1)\2 + Y\10 − Y\200 668.5950 sols 6801–8400 (Y − 1)\2 + Y\10 − Y\300 668.5967 sols 8401–10000 (Y − 1)\2 + Y\10 − Y\600 668.5983 sols
This extended intercalation scheme gives an average year of 668.59453 days over a 10,000-year period: this results in an error of only about one sol at the end of 12,000 Martian years, or the year 24,180 of the Common Era.Gangale, Thomas. (2006-07-01). "The Architecture of Time, Part 2: The Darian System for Mars." Society of Automotive Engineers. SAE 2006-01-2249.
Calendar layout The year is divided into 24 months. The first 5 months in each quarter have 28 sols, while the final month has 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 and can also be rationalised as making the average length of the Martian week close to the average length of the Terrestrial week; 28 Earth days is very close to Martian sols, whereas a month is an average length of Martian sols.
In the table, the sols of the week are Sol Solis, Sol Lunae, Sol Martis, Sol Mercurii, Sol Jovis, Sol Veneris, Sol Saturni.
Sagittarius Dhanus Capricornus So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 28 Makara Aquarius Kumbha So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 Pisces Mina Aries So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 28 Mesha Taurus Rishabha So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 Gemini Mithuna Cancer So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 28 Karka Leo Simha So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 Virgo Kanya Libra So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 28 Tula Scorpius Vrishika So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa So Lu Ma Me Jo Ve Sa 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 8 9 10 11 12 13 14 8 9 10 11 12 13 14 15 16 17 18 19 20 21 15 16 17 18 19 20 21 15 16 17 18 19 20 21 22 23 24 25 26 27 28 22 23 24 25 26 27 28 22 23 24 25 26 27 28
The last sol of Vrishika is an intercalary sol that only occurs on leap years, like February 29 in the Gregorian calendar.
Start of year The Martian year is treated as beginning near the equinox marking spring in the northern hemisphere of the planet. Mars currently has an axial inclination similar to that of the Earth, so the Martian seasons are perceptible, though the greater eccentricity of Mars's orbit about the Sun compared with that of the Earth means that their significance is strongly amplified in the southern hemisphere and masked in the northern hemisphere.
Epoch Gangale originally chose late 1975 as the epoch of the calendar in recognition of the American Viking program as the first fully successful (American) soft landing mission to Mars (the earlier 1971 Soviet Mars 3 Landing having delivered only 15 seconds of data from the planet's surface). In 2002 he adopted the Telescopic Epoch, first suggested by Peter Kokh in 1999 and adopted by Shaun Moss in 2001 for his Utopian Calendar, which is in 1609 in recognition of Johannes Kepler's use of Tycho Brahe's observations of Mars to elucidate the laws of planetary motion, and also Galileo Galilei's first observations of Mars with a telescope. Selection of the Telescopic Epoch thus unified the structures of the Darian and Utopian calendars, their remaining differences being nomenclatural. It also avoids the problem of the many telescopic observations of Mars over the past 400 years being relegated to negative dates.
Nomenclature The Darian calendar has been widely imitated. Suggested variations abound on the internet that use different nomenclature schemata for the days of the week and the months of the year. In the original Darian calendar, the names of the 24 months were provisionally chosen by Gangale as the Latin names of constellations of the zodiac and their Sanskrit equivalents in alternation. The 7 sols of the week, similarly, were provisionally named after the Sun, the largest Martian moon Phobos (Sol Phobotis) and the 5 brightest planets as seen from Mars including Earth (Sol Terrae). These were later modified to follow the familiar convention of the Romance languages, replacing Sol Phobotis with Sol Lunae and Sol Terrae with Sol Martis. The Darian Defrost Calendar, however, uses the Rotterdam System to create new names for the Martian months out of patterns relating letter choice and name length to month order and season. The Utopian Calendar, devised by the Mars Time Group in 2001, also has additional suggestions for nomenclature modification. In 2025, Gangale adapted the Chinese solar terms as alternative names of the months to accommodate East Asian cultures, observing, "they invoke events which may occur with the passing of the seasons on Mars someday if terraforming is successful; thus, the Darian Chinese system is both aspirational and inspiring." +Comparison of month names by systemN°DarianRotterdamUtopianChinese1SagittariusAdirPhoenix火星春分 (Spring Equinox)2DhanusBoraCetus火星清明 (Pure Brightness)3CapricornusCoanDorado火星穀雨 (Grain Rain)4MakaraDetiLepus火星立夏 (Beginning of Summer)5AquariusEdalColumba火星小滿 (Grain Buds)6KumbaFloMonoceros火星芒種 (Grain in Ear)7PiscesGeorVolans火星夏至 (Summer Solstice)8MinaHelibaLynx火星小暑 (Minor Heat)9AriesIdanonCamelopardalis火星大暑 (Major Heat)10MeshaJowaniChamaeleon火星立秋 (Beginning of Autumn)11TaurusKirealHydra火星處暑 (End of Heat)12RishabhaLarnoCorvus火星白露 (White Dew)13GeminiMediorCentaurus火星秋分 (Autumn Equinox)14MithunaNeturimaDraco火星寒露 (Cold Dew)15CancerOzulikanLupus火星霜降 (Frost's Descent)16KarkaPasurabiApus火星立冬 (Beginning of Winter)17LeoRudiakelPavo火星小雪 (Minor Snow)18SimhaSafundoAquila火星大雪 (Major Snow)19VirgoTiunorVulpecula火星冬至 (Winter Solstice)20KanyaUlasjaCygnus火星小寒 (Minor Cold)21LibraVadeunDelphinus火星大寒 (Major Cold)22TulaWakumiGrus火星立春 (Beginning of Spring)23ScorpiusXetualPegasus火星雨水 (Rain Water)24VrishikaZungoTucana火星驚蟄 (Awakening of Insects)
Mars Julian sol The Mars Julian sol count is analogous to the Julian Day count on Earth, in that it is a continuous numerical counting of days from an epoch. The Mars Julian sol epoch is the same as for the Darian calendar, thus Mars Julian sol 0 is 1 Sagittarius 0.
Comparison with timekeeping systems in planetary science