The Martian Calendar

The Martian calendar is designed for long-term human life on Mars and uses MTC, or Mars Coordinated Time, while the calendar itself follows a year-week-day structure. The calendar epoch is set to 13 December 2048, treated as Year 1, Week 1, Day 1, marking the first crewed landing on Mars.

The calendar itself was chosen diplomatically by China to commemorate its historic landing and to establish a new system not tied different historical calendars previously used. That included not only the starting date, but also the structure of the year and the naming of the sols. As part of the compromise, China kept Yínghuò, an ancient Chinese name for Mars, as one of the sol names, while the rest were agreed as broader astronomical and ceremonial names that could be accepted internationally. The final result still leaned somewhat Western, but it was accepted as a practical compromise by the major powers and became the first common civil calendar of Mars.

How time is structured

The sol

Martian clock time is meant to feel familiar. It still uses a normal 24-hour format, with 60 minutes per hour and 60 seconds per minute. The difference is that a Martian day, called a sol, is slightly longer than an Earth day.

One sol lasts 24 hours, 39 minutes, and 35 seconds in Earth time. Mars therefore does not use strange new hours or minutes. It keeps the same basic clock structure, but stretches it across a slightly longer day. That means Martian clock time slowly slips against Earth time from one day to the next.

The week

The Martian week contains 10 sols. This gives the calendar a broader rhythm than the 7-day week used on Earth and makes it easier to organize the long Martian year into regular blocks of time.

The year

A Martian year is divided into 67 weeks. The first 66 weeks always contain 10 sols each. The 67th and final week is shorter, with 8 sols in a common year and 9 sols in a leap year.

That gives the calendar two possible year lengths:

• Common year: 668 sols
• Leap year: 669 sols

Most of the year therefore follows a simple and repetitive pattern, with only the last week changing length. The result is a calendar that is easy to read and practical to use, while still staying close to the real length of the Martian year.

The sol names

The 10-sol week uses ceremonial and celestial names instead of borrowing directly from Earth’s weekday tradition. These names begin with the Sun and then move outward through the Solar System, ending beyond it with Andromeda.

• Solaris — from Latin solaris, meaning “of the sun” from sol = sun. Marks the start of the Martian week.
• Mercurion — derived from Mercury, the Roman name of the first planet from the Sun and the god associated with speed and movement.
• Vesper — from Latin vesper, meaning evening, and associated in classical use with the evening star, Venus seen after sunset.
• Erda — a name tied to Earth, used here as the sol that recalls humanity’s world of origin. In the Martian calendar, Erda stands for memory, departure, and the living bond between Mars and Earth.
• Phobon — derived from Phobos, one of Mars’s two moons. Phobos comes from Greek and means fear or panic, linked to the martial world of Ares.
• Deimon — derived from Deimos, Mars’s other moon. The original Greek name carries the sense of dread or terror.
• Yínghuò (熒惑) — an ancient Chinese name for Mars. The term carries the sense of flickering, dazzling, or troubling light. A direct link to China’s historic role in Mars exploration.
• Jovian — derived from Jove or Jupiter, the great sky god of Roman tradition.
• Saturnis — a constructed ceremonial name derived from Saturn, the Roman name of the sixth planet from the Sun.
• Andromedan — inspired by Andromeda, the nearest major galaxy to the Milky Way. A forward-looking tone that looks toward humanity's interplanetary civilization future.

Holidays

In addition to its ordinary week and year structure, the Martian calendar also includes a small number of major annual observances. These are not only days of rest or ceremony, but markers of memory, season, and identity in a society that lives permanently away from Earth.

New Year

New Year marks the anniversary of the first human landing on Mars. In calendar terms, it is the beginning of each new Martian year. In cultural terms, it is the founding day of human Mars history.

Unlike many older calendars, this New Year is not based on an abstract astronomical reset alone. It is tied to a human event: arrival. It therefore carries a strong historical meaning. It commemorates landing, survival, and the moment when Mars ceased to be only a destination and became a place of settlement.

As a public observance, New Year is likely to combine ceremony and remembrance. It is the day on which the colony looks backward to the first landing and forward to the year ahead.

Midvinter

Midvinter is the principal winter observance of the northern Martian year. It falls at the deep center of winter and marks endurance through the coldest and darkest phase of the seasonal cycle.

Its meaning is practical as much as ceremonial. On Mars, winter is not only symbolic. It affects light, power, outdoor operations, and the psychological shape of daily life. Midvinter therefore becomes a festival of shelter, stored warmth, discipline, and shared resilience.

Midsommar

Midsommar is the principal summer observance of the northern Martian year. It falls near the height of summer and marks light, openness, and the most favorable part of the year for movement and activity on the surface.

Where Midvinter emphasizes endurance and shelter, Midsommar emphasizes expansion and vitality. It belongs to the season of travel, exploration, construction, and fieldwork. A time to be outside, to be active, and go further afield.

Terra Nordica Sol

Terra Nordica was founded on 9 April 2057 (UTC), when Expedition 4 landed in Kasei Valley and established a permanent Nordic presence on Mars. Earlier expeditions had surveyed and constructed the settlement, but Expedition 4 brought its first permanent residents. In the Martian calendar, the date corresponds to MY 5, Week 29, Sol 4 (Erda). Terra Nordica has been continuously inhabited ever since.

Leap years and drift

Mars does not orbit the Sun in an exact whole number of sols. Using a mean sidereal orbital period of 686.98 Earth days and a sol length of 1.027 Earth days, the Martian year comes out to about 668.60 sols.

Source: NASA JPL Solar System Dynamics, Horizons System. Horizons Mean sidereal orbital period value used: 686.98 Earth days.

Because a calendar year must contain a whole number of sols, this system alternates between 668-sol common years and 669-sol leap years. The leap-year rule is simple: every 5-year cycle contains 3 leap years.

That average is extremely close to the astronomical value. The difference is only:

At that rate, the calendar drifts by about one sol in 1,579 Martian years, which is about 2,970 Earth years.