Time on Mars is easily divided into days based on its rotation rate and years based on its orbit. Sols, or Martian solar days, are only 39 minutes and 35 seconds longer than Earth days, and there are 668 sols (687 Earth days) in a Martian year.
For convenience, sols are divided into a 24-hour clock. Each landed Mars mission keeps track of "Local Solar Time," or LST, at its landing site, because Local Solar Time relates directly to the position of the Sun in the sky and thus the angle from which camera views are illuminated. The time of day, Local Solar Time, depends upon the lander's longitude on Mars.
Unlike on Earth, there is no leisurely-orbiting moon to give Mars "months," and while there have been many imaginative calendars suggested for Mars, none is in common use. The way that scientists mark the time of Mars year is to use solar longitude, abbreviated Ls (read "ell sub ess"). Ls is 0° at the vernal equinox (beginning of northern spring), 90° at summer solstice, 180° at autumnal equinox, and 270° at winter solstice.
On Earth, spring, summer, autumn, and winter are all similar in length, because Earth's orbit is nearly circular, so it moves at nearly constant speed around the Sun. By contrast, Mars' elliptical orbit makes its distance from the Sun change with time, and also makes it speed up and slow down in its orbit. Mars is at aphelion (its greatest distance from the Sun, 249 million kilometers, where it moves most slowly) at Ls = 70°, near the northern summer solstice, and at perihelion (least distance from the Sun, 207 million kilometers, where it moves fastest) at Ls = 250°, near the southern summer solstice. The Mars dust storm season begins just after perihelion at around Ls = 260°.
The coincidence of aphelion with northern summer solstice means that the climate in the northern hemisphere is more temperate than in the southern hemisphere. In the south, summers are hot and quick, winters long and cold.
For the purpose of this comparison, we use the solar longitude range 0°-360° to define a Mars year and adopt April 11, 1955 (Ls=0°) as the beginning of year 1. In this arbitrary convention, the Mariner 9, Viking, Phobos, and Pathfinder missions occurred in years 9-10, 12-15, 19-20, and 23, respectively. By comparison, the 1992-1999 [Earth-based] millimeter observations extend over years 21-24, and the 1997-1999 [Mars Global Surveyor] TES observations extend over years 23 and 24. (Clancy et al., 2000)
They picked Year 1 to correspond with the year of a global dust storm widely observed in 1956. A more recent paper defined the existence of a Mars Year 0 (starting on May 24, 1953), and defined previous years as having negative numbers (Piqueux et al., 2015).