The Martian Calendar
Anyone living on Mars will measure time differently than people measure it on Earth. Days are almost 45 minutes longer on Mars and it takes approximately two Earth years for Mars to make one trip around the sun. In addition, the moon plays a huge role in the way humans measure time. After all, a month on Earth is based on the phases of our moon.
NASA teams today supporting missions to Mars generally work on Martian clocks based on the longer Martian days—called “sols”. If you’ve seen or read “The Martian”, you may have noticed this in use. When people say the word “sol”, the specifically mean a day on Mars.
Every calendar must have an “epoch date”—the date at which counting starts. Each mission has its own “epoch date” which is the mission touches down on Mars. That date is sometimes called “Sol 0” and sometimes “Sol 1”. Either way, each Mars mission historically works on its own calendar.
More generally, for Mars, the epoch date is considered the spring equinox of the northern hemisphere on Mars during the Earth year of 1955—April 11, 1955. So, the first day of year 1 of our Martian calendar equates to April 11, 1955. January 1, 2019 thus takes place late in the Martian year 34.
That much is pretty standard when it comes to Martian calendars. However, there is no commonly accepted way to divide up a Martian year. Our basis for dividing up a year on Earth—the cycles of the moon—is nonsensical in a Martian context. One Martian moon crosses the sky several times a day and the other is too faint to be meaningful to any people living on Mars.
Nevertheless, the first Martian colonists would be psychologically accustomed to tracking time in Earth months and a Martian year would feel way too long for marking anniversaries. Our Martian calendar is thus divided into two halves of 10 months each. The calendar to the right shows the eighth month in the second half of Martian year 34. The 19th day of that month corresponds to New Year's Day 2019 on Earth.
The short form for a Martian date is requires four numbers instead of the three we use: year, 1 or 2 for half year, month, day. 2018-01-01 is thus 0034-2-8-19 (Octosis is the eighth month). We’ll sometimes show it as 34-2-8-19 as well.