Summary
A year on Mars is 687 Earth days or 668.59 sols. A sol is the name for a day on Mars, and is only slightly longer than an Earth day, being 24 hours, 39 minutes, and 35 seconds long. So a Martian year is equal to 1.88 Earth years, or 1 year, 320 days, and 18.2 hours. Martian clocks use the same hours, minutes and seconds as on Earth, however they have a additional 39 minutes and 35 seconds.
Mars has time zones defined at regular intervals from the prime meridian, as on Earth.
Martian dates (mDate) are expressed simply the day number preceeded by an m, for example, m345. There are no months as such. Some people then add the terran year, so a complete date would be m345 2073. Others use the martian year (mYear), which by convention uses December 29, 1873 as year 1, day 1, so using this system a complete date would look like m345, 200
Because of Mars's axial tilt of 25.19 degrees, is similar to the axial tilt of the Earth, we have seasons like the Earth, though on Mars they are nearly twice as long.
Time of day
The average length of a Martian sidereal day is 24h 37m 22.663s in terms of Earth hours, and the length of its solar day is 24h 39m 35.244s (the latter is known as a sol, more precisely 88,775.24409 seconds). The corresponding values for Earth are 23h 56m 04.2s and 24h 00m 00.002s, respectively. This yields a conversion factor of 1.027346 sols/day. Thus Mars's solar day is only about 2.7% longer than Earth's.
Coordinated Mars Time (MTC)
MTC is a Mars analog to Universal Time (UT) on Earth. It is defined as the mean solar time at Mars's prime meridian (i.e., at the centre of the crater Airy-0). The name "MTC" is intended to parallel the Terran Coordinated Universal Time (UTC), but this is somewhat misleading: what distinguishes UTC from other forms of UT is its leap seconds, but MTC does not use any such scheme. MTC is more closely analogous to UT1. The term "MTC" as the name of a planetary standard time for Mars first appeared in the Mars24 sunclock coded by the NASA Goddard Institute for Space Studies.
Timezones
Mars has a prime meridian, defined as passing through a small crater Airy-0. Mars has 24 time zones defined at regular hourly intervals from the prime meridian, as on Earth.
Sols
The term sol is used by to refer to the duration of a solar day on Mars. A mean Martian solar day, or "sol", is 24 hours, 39 minutes, and 35.244 seconds
Terran astronomers often prefer to use Julian dates for timekeeping purposes. This is simply a sequential count of days, bypassing the complications of calendars. The counterpart on Mars was originally known as the Mars Sol Date, or MSD, which is a running count of sols since approximately December 29, 1873. Any start date could be used; however, it should be far enough in the past that all historically recorded events occur after the start. Overtime these became known as mDays
Mars Sol Date
The Mars Sol Date is defined mathematically as MSD = (Julian date using International Atomic Time - 51549.0 + k)/1.02749125 + 44796.0, where k is a small correction of approximately 0.00014d (or 12sec) due to uncertainty in the exact geographical position of the prime meridian at Airy-0 crater.
The word "yestersol" was coined during early Mars operations to the previous sol (the Mars version of "yesterday") and came quickly into fairly wide use. Other neologisms such as "tosol" (for "today") and "nextersol" or "morrowsol" (for "tomorrow") were less successful.
The Calendar and mDates
Mars scientists typically keep track of the Martian year by use of the heliocentric longitude (or "seasonal longitude"), typically abbreviated Ls, related to the position of Mars in its orbit around the Sun. Ls is defined as 0 degrees at the Martian northward equinox, and hence is 90 degrees at the Martian northern solstice, 180 at the Martian southward equinox, and 270 degrees at the Martian southern solstice.
For most day-to-day activities on Earth, people don't use Julian days, but the Gregorian calendar, which despite its various complications remains useful. It allows for easy determination of whether one date is an anniversary of another, whether a date is in winter or spring, and what is the number of years between two dates. This is much less practical with Julian days count. For this reasons, Mars settled on mDates, based on a sequential count of days like the Mars Sol Date, starting from the same arbitary day, December 29, 1873. Every 668 Sols counts as a matian year, so a full date would be m345 200.
For the Gregorian (Earth) calendar, the leap-year formula (intercalculation) is every 4th year except for every 100th year except for every 400th year, which produces an average calendar year length of 365.2425 solar days, close to the Earth equinox year.
On Mars, a scheme for leap years was be needed. technically most years would be leap years since the fractional sol, the remainder of a sol left each year after a whole number of days has passed, is more than 0.5. The selected intercalculation is having a leap year every odd year or year ending in 0 except every 100th year, except every 500th year. This gives us an average year of 668.592 sols, which would be pretty perfect for the mean tropical year (average of all seasons).
The main other proposal was the Darian calendar. It has 24 "months", to accommodate the longer Martian year while keeping the notion of a "month" that is reasonably similar to the length of an Earth month.
Martian year
This length of time for Mars to complete one orbit around the Sun is its sidereal year, and is about 668.5991 sols or 686.98 Earth solar days. Because of the eccentricity of Mars' orbit, the seasons are not of equal length. Assuming that seasons run from equinox to solstice or vice versa, the season Ls 0 to Ls 90 (northern-hemisphere spring / southern-hemisphere autumn) is the longest season lasting 194 Martian sols, and Ls 180 to Ls 270 (northern hemisphere autumn / southern-hemisphere spring) is the shortest season, lasting only 142 Martian sols.
Formula to convert UTC to MTC
MTC = (seconds since 2000-01-06 00:00:00 UTC)×(86400/88775.244)) + 44795.9998
