EXODUS CALENDAR FOR MARS

Revision 2026.01.05

INTRODUCTION

An accurate and user-friendly Martian calendar would be invaluable for future Martian colonists, yet no commonly accepted system exists today. Current proposals, such as the Darian calendar [1], may appear overly complex and demand memorization of numerous unfamiliar terms. The ideal Martian calendar should combine both accuracy and simplicity, retaining familiar month and weekday names. Additionally, maintaining some degree of backwards compatibility with existing Martian timekeeping solutions would be highly beneficial.

DEFINITIONS

• The duration of Martian solar day (“sol”), that is, the time for the plane to rotate around its axis with respect to the Sun is currently 24 hours 39 minutes and 35.244 seconds (88775.244 seconds, equal to 1.02749125 Earth solar days)

• The duration of Martian tropical year (orbital period with respect to the Sun) is 668.5921 sols, but for the planetary calendar purposes, time between vernal equinoxes is often used, as in Gregorian calendar, allowing more close alignment with seasons of the year. In Mars case, this value is equal to 668.5907 sols [1, p. 3]

DESCRIPTION

I propose the following scheme:

Using the northward equinox year (668.5907 sols) as the reference year length, this calendar operates on a 22-year cycle:

• Ten 668-sol years (even-numbered years: second, fourth, sixth, etc.)
• Eleven 669-sol years (odd-numbered years: first, third, fifth, etc.)
• One 670-sol year (leap year)

In one sentence, rephrased: odd years are 669 sols, even years are 668 sols except when divisible by 22, then they are 670 sol long.

Each year comprises 12 months bearing the same names as their terrestrial counterparts, with each containing 56 sols except December, which varies in length: 52, 53, or 54 sols. Weeks maintain the familiar seven-day structure with Earth’s weekday names (Monday, Tuesday, etc.), and each month contains exactly eight weeks. All months begin on Monday and conclude on Sunday, with only December's final week ending on Wednesday, Thursday, or Friday, depending on the year's length. Every new year begins on Monday.

This concept requires minimal new information to memorize: essentially just the 22-year cycle structure and two month lengths: one constant and one dependent on the year's position within the cycle.

EPOCH

The epoch date and time is provisionally set to Mars vernal equinox of April 11, 1955, at UTC 19:21:51 to keep the calendar aligned with planetary seasons and in sync with MST (Mean Solar Time). Years are counted from 1, and negative years start from -1, as in Gregorian calendar (i.e. there is no year 0).

ACCURACY

Calendar Year Length = (668 x 10 + 669 x 11 + 670)/22

This yields an average calendar year duration of 668.5909(09) sols, creating a naive error estimate of 0.00021 sols per year, comparable to the Gregorian calendar's 0.00013-day annual discrepancy. If no other factors take into account, such a system would remain reasonably accurate for the foreseeable future, accumulating an error of only 1 sol after approximately 4,782 Martian years, assuming fixed duration of Mars northward equinox year. But as the Martian year length is known to drift (+0.00079 sols per 1,000 Martian years [1, p3]), more accurate error expression that includes annual duration drift can be expressed as following:

$$ Accumulated\ Error(sols) = \int_0^t \left(\frac{668 \times 10 + 669 \times 11 + 670}{22} - (668.5907 + 0.00079t/1000)\right) dt $$

Solving for t, error expression becomes:

$$ 2.1 \times 10^{-4}𝑡 - 3.95 \times 10^{-7}𝑡^2 $$

So 1 sol error will accumulate in 1878 Martian years (approximately 3532 Earth ones), illustrated below:

SEASONS

The seasonal advance of the Sun at Mars is commonly measured in terms of the areocentric longitude (or Solar Longtitude Angle) Ls, as referred to the planet's vernal equinox (the ascending node of the apparent seasonal motion of the Sun on the planet's equator). As defined, Ls = 0°, 90°, 180°, and 270° indicate the Mars northern hemisphere vernal equinox, summer solstice, autumnal equinox, and winter solstice, respectively.[2, 3]

Ls range	Sol range	Dates (approx.)	Notes
0-30	0.0-61.2	Jan 01 - Feb 06	Northern Spring Equinox at Ls=0
30-60	61.2-126.6	Feb 06 - Mar 15
60-90	126.6-193.3	Mar 15 - Apr 26	Aphelion (largest Sun-Mars distance) at Ls=71
90-120	193.3-257.8	Apr 26 - May 34	Northern Hemisphere Summer Solstice at Ls=90
120-150	257.8-317.5	May 34 - Jun 38
150-180	317.5-371.9	Jun 38 - Jul 36
180-210	371.9-421.6	Jul 36 - Aug 30	Autumn Equinox at Ls=180, Dust Storms begin
210-240	421.6-468.5	Aug 30 - Sep 21
240-270	468.5-514.6	Sep 21 - Oct 10	Perihelion (smallest distance) at Ls=251
270-300	514.6-562.0	Oct 10 - Nov 03	Northern hemisphere Winter Solstice at Ls=270
300-330	562.0-612.9	Nov 03 - Nov 53
330-360	612.9-668.6	Nov 53 - EOY	Dust Storm Season ends

SOURCE CODE

In addition to PyPi package source, there are some command-line utilities in "/tools" folder of GitHub repository - one for conversions between terrestrial (UTC) and Martian (in MTC) dates ("exodus.py"), accuracy test ("accuracy.py") https://github.com/DarkStar1982/exodus_calendar/

INSTALLATION

Run 'pip install exodus-calendar', latest version recommended. Tested for Python 3.10-3.13.

USAGE

Main functions of note in exodus_orbitals.utils:

• earth_datetime_to_mars_datetime(input_date, mars_sec_on). Converts input_date in UTC (as timezone-aware datetime object) to Martian date time as a tuple of (date, time, weekday, Ls), where Ls is solar longitude angle. More details about this value available in [2], [3] and [4].

• mars_datetime_to_earth_datetime(input_date, mars_sec_on) Converts Martian timestamp (as string) to Earth one in UTC (as timezone-aware datetime object)

• mars_datetime_now(format, mars_sec_on) Prints Martian timestap as string (format="str", default value) or milliseconds (format="ms") since calendar epoch

"mars_sec_on" parameter allows to use either standard second (1000 ms) when False or Martian second (1027.5 ms) when True for more convienient 24-hour timekeeping. When used, the time returned will be in sync with (unofficial) MTC timezone - time at zero Martian meridian, Mars equivalent to UTC. Set to False by default.

PUBLICATION

Silin, D.V. (2025). The Exodus Calendar: A Simple Timekeeping System for Martian Colonists. Journal of the British Interplanetary Society, Vol 78 No 09 – September 2025.

https://bis-space.com/shop/product/jbis-vol-78-no-09-september-2025/

WEB DEMO

https://marscalendar.space/

SUPPORT PROJECT DEVELOPMENT

https://www.paypal.com/paypalme/DenysSilin

REFERENCES

1. Gangale, Thomas. (2006). The Architecture of Time, Part 2: The Darian System for Mars. SAE Technical Papers. 10.4271/2006-01-2249.

2. https://www.giss.nasa.gov/tools/mars24/help/notes.html

3. https://www-mars.lmd.jussieu.fr/mars/time/solar_longitude.html

4. Michael Allison, Megan McEwen (2000). A post-Pathnder evaluation of areocentric solar coordinates with improved timing recipes for Mars seasonal/diurnal climate studies. Planetary and Space Science 48, 15-235.