The idea in one sentence
A calendar made to measure for Mars: it counts Martian days (sols), groups them into 18 named months, and converts any instant to Earth (UTC) dates and back.
It is called the Martian Horizon Time Standard (MH-STD-001). It is a civil calendar: it organises life and work on Mars (missions, rovers, future settlers) the way the Gregorian calendar does on Earth. Everything on this page comes from a few simple rules, explained one by one below.
1 sol24 h 39 min 35,244 s
18months
668–670sols per year
7sols per week
The sol and Martian time
A Martian day is called a sol and lasts 24 h 39 min 35.244 s (88,775.244 SI seconds): the time Mars takes to turn once relative to the Sun. It is 2.75 % longer than an Earth day; in other words, 1 sol = 1.0274912527 Earth days.
Martian time (MTC, Coordinated Mars Time) splits the sol into 24 hours of 60 minutes of 60 seconds, as on Earth. The only difference is that each Martian second lasts 1.0275 Earth seconds. That way the Martian clock does a full turn and reads 00:00:00 exactly when a new sol begins.
That is why the two clocks on the home page and in “Today on Mars” do not tick alike: the Martian one falls slightly behind every second, and each sol it ends up about 39 minutes 35 seconds behind!
The year and the months
A Martian year (the time Mars takes to orbit the Sun) lasts about 686.97 Earth days, nearly two Earth years. In sols that is about 668.6. A calendar needs whole days, so the year has 668, 669 or 670 sols following a leap-year rule (next section).
The year is divided into 18 months, labelled with the letters A to R (none skipped). The length rule is simple:
- A normal month has 37 sols.
- Two fixed months, Elysio (5th, letter E) and Nilo (14th, letter N), always have 38 sols. They are called “pillar months”.
- Only two months change length: Acidalio (1st) and Juveno (10th), which receive an extra sol in certain years.
Check: 18 × 37 = 666, plus 1 extra sol in each pillar month (2) = 668 sols, the base year. A year is always the sum of its 18 months.
Years of 669 and 670 sols (leap years)
Because the real year lasts 668.59 sols rather than 668, extra sols must be added now and then so the calendar does not drift from the seasons. There are three rules and they add up:
| Rule | What happens | Adds on average |
|---|
| Odd year | +1 sol in Juveno (38 sols that year) | +0,5 |
| Year multiple of 11 | +1 sol in Acidalio (38 sols that year) | +0,0909 |
| Year multiple of 800 | +1 sol in Juveno | +0,00125 |
Result: an even year that is neither a multiple of 11 nor of 800 has 668 sols; odd (or multiple of 800) gives 669; odd and also a multiple of 11 (like year 11) gives 670. Year 22, even and a multiple of 11, has 669. The 800 rule never overlaps the odd-year rule, because 800 is even.
Average: 668 + 0.5 + 0.0909 + 0.00125 = 668.5921 sols per year, very close to Mars’s real tropical year (≈ 668.599). The gap is about 0.007 sols per year: the calendar would drift one sol roughly every century and a half, a tiny error for a civil calendar.
The 18 names and why they are called that
Months follow the alphabet from A to R, so the order is self-evident: from a letter you know where you are in the year. Each name comes from the International Astronomical Union (IAU) nomenclature: a Martian landform, one of its two moons or a classical figure tied to Martian cartography, whose name starts with that letter and is adapted to the shape of a month name. Here is the reason behind each one.
Names are never translated: they are exactly the same in Spanish, English or any other language, like proper nouns. A “sol 12 of Olympo” therefore means the same to everyone.
Why each name
A Acidalio · Acidalia Planitia
Acidalia Planitia is a very large plain in the northern lowlands of Mars. Its name comes from Acidalia, a spring in classical mythology linked to Venus, which already appeared on Schiaparelli’s telescopic maps in the nineteenth century.
B Boreo · Planum Boreum (north polar cap)
Planum Boreum is the plateau around the north pole of Mars, covered by a cap of water ice with layers of dust. The name comes from Boreas, the north wind in Greek mythology.
C Chrysio · Chryse Planitia (Viking 1)
Chryse Planitia, the “plain of gold” (from the Greek chrysé), is a large plain where the Viking 1 probe landed on 20 July 1976. The original spelling of the name is kept.
D Deimo · Moon Deimos
Deimos is the smaller and more distant of Mars’s two moons; Asaph Hall discovered it in 1877. Its name is that of Deimos, the Greek personification of dread and a companion of Ares.
E Elysio · Elysium Planitia
Elysium Planitia is a region of volcanic plains, the second great volcanic area of Mars after Tharsis. The name comes from the Elysian Fields (Elysion) of Greek mythology. It is one of the two months with a fixed 38 sols.
F Fortuno · Fortuna Fossae
Fortuna Fossae is a set of fossae, long narrow depressions formed by faults in the crust. The name comes from Fortuna, the Roman goddess of luck.
G Gangio · Ganges Chasma
Ganges Chasma is a canyon about 4 km deep in the eastern part of Valles Marineris. Its name comes from the river Ganges, through an old nineteenth-century albedo name.
H Hespero · Hesperia Planum
Hesperia Planum is a volcanic plateau northeast of the Hellas basin. Its name comes from Hesperos, the evening star, and it gives its name to the Hesperian period of Mars’s geological history.
I Isido · Isidis Planitia
Isidis Planitia is an impact basin about 1,500 km across, on the boundary between the southern highlands and the northern lowlands. It is named after the Egyptian goddess Isis.
J Juveno · Juventae Chasma
Juventae Chasma is a canyon north of Valles Marineris with sulphate deposits. Its name comes from Juventas, the Roman goddess of youth. In the calendar, it is the month that gains an extra sol in odd years and in multiples of 800.
K Kaseo · Kasei Valles
Kasei Valles is a system of large outflow channels, about 2,400 km long. Its name comes from kasei (火星), the Japanese word for the planet Mars.
L Luno · Lunae Planum
Lunae Planum is a basalt plateau north of Valles Marineris, with compressional ridges. It takes its name from an old albedo feature, Lunae Lacus, the “lake of the Moon”, on telescopic maps.
M Mangalo · Mangala Valles
Mangala Valles is an outflow channel several hundred kilometres long, carved by large floods. Mangala is the Sanskrit name for the planet Mars (मंगल).
N Nilo · Nili Fossae
Nili Fossae is a set of curved fossae northwest of the Isidis basin, with clays, carbonates and olivine. Its name comes from the Nile, through an old albedo name. It is the other month with a fixed 38 sols.
O Olympo · Olympus Mons
Olympus Mons is the largest known volcano in the Solar System: about 22 km high and about 600 km wide. It is named after Mount Olympus, the home of the Greek gods.
P Phobo · Moon Phobos
Phobos is the larger and closer of Mars’s two moons: it orbits about 6,000 km above the surface, faster than the planet turns, and crosses the sky several times per sol. Its name is that of Phobos, the Greek personification of fear; Asaph Hall also discovered it in 1877.
Q Quirino · Quirinus (Roman deity, civil order)
Quirinus is the name of an ancient Roman deity, linked to the community of citizens (the quirites) and related to Mars. Unlike the other months, it does not correspond to a geographic feature but to a name from Roman tradition.
R Ravio · Ravi Vallis
Ravi Vallis is an outflow channel that starts at the chaotic region of Aromatum Chaos. Its name comes from the river Ravi, in India and Pakistan.
The week and “coincidence days”
The Martian week has 7 sols and is continuous: it never restarts at a new month or year, just like on Earth. So a 37-sol month starts on a different weekday each time, and the grid rows do not always start on the same day.
Because the sol is almost 40 minutes longer than an Earth day, Mars and Earth clocks slowly drift apart. Every ≈ 37.375 Earth days (≈ 36.37 sols) they read the same time again: that is the coincidence day. In the calendar those sols glow with a blue halo, and tapping them shows the exact UTC time they match.
The reference date and UTC
To convert between Earth and Martian dates we need a common starting point. The standard uses this one: on 6 January 2000 at 00:00:00 UTC the Martian clock also reads 00:00:00, and that date is sol 6 of the month Olympo (15th) in year 24. It is a fixed anchor that never changes, and everything else is computed from it, forwards and backwards.
Counting backwards from there, year 1 starts on 11 April 1955, the start of “Mars Year 1” used by scientists. Our year numbering therefore matches what you will see in NASA and ESA papers and missions (year 39 began at the end of September 2026).
All dates are stored in UTC (Earth’s universal time). The Martian date is never stored: it is computed when shown, from the UTC. That way two dates can never contradict each other and every calculation can be checked.
Transparency: the anchor is a convention of the standard. The scientists’ “official” Martian time (astronomical MTC) uses a different reference and can differ from ours by about 15–21 seconds. For a civil calendar that is irrelevant, and a round, memorable reference was preferred.
Seasons and solar longitude (Ls)
Mars has seasons like Earth because its axis is tilted about 25°. Scientists measure them with the solar longitude (Ls), an angle from 0° to 360° showing where Mars is in its orbit: Ls 0° = start of spring (northern hemisphere), 90° = summer, 180° = autumn and 270° = winter.
Unlike Earth, Martian seasons differ in length (roughly 154 to 194 sols) because the orbit is quite elongated: Mars moves faster near the Sun (perihelion) and slower far away (aphelion). Both points are marked on the year wheel. The big dust-storm season usually falls between Ls 180° and 360°, when Mars is closest to the Sun.
The seasons and Ls shown here are computed from orbital elements: a very good approximation, not an official ephemeris-grade measurement.
How to use this page
- Today on Mars: the clock, the Martian and Earth date right now, the season and how far through the Martian year we are.
- The wheel: the whole year. Each arc is a month (its size is proportional to its sols), the outer ring shows seasons and dots are events. Tap an arc to jump to that month.
- The month: each cell is a sol; its Earth (UTC) date is at the bottom left. Coloured dots are events, the orange border is today and the blue glow a coincidence day. Navigate with the buttons, keyboard arrows or by swiping.
- Sol details: tap a sol to see its start and end in UTC, the season, events and links to missions or stories; you can copy the link or add it to your calendar (.ics).
- Coming up: upcoming events counting down in sols and Earth days. Those labelled “calculated” (oppositions, conjunctions, launch windows) are computed by the page itself.
- Travel to a date: type an Earth date and it takes you to its sol, or pick a Martian year, month and sol. For finer conversions use the Converter.
Frequently asked questions
Why 18 months and not 12 or 24?
Eighteen months of about 37 sols give a 668-sol year with similar-sized months, each close to an Earth month (37 sols ≈ 38 Earth days). And the alphabet up to R gives every month its own name.
Why do Elysio and Nilo have 38 sols?
18 months of 37 sols add up to 666 and the base year needs 668: the two missing sols go to two fixed months, spread through the year (5th and 14th) to keep it balanced.
Is there a sol 0 or a month 19?
No. Sols are counted from 1 and only the 18 months A–R exist. Any other value is rejected.
Will the calendar ever change?
Rules and reference date are fixed. If they were ever updated it would be announced explicitly; recorded dates would not change because they are stored in UTC.
