Our Martian calendar consists of 24 months, alternating between 28 and 27 sols (Martian days), for a total of 660 sols in a standard year. To stay roughly aligned with the astronomical Mars year, which is about 668.6 sols long, we introduce a leap sol every 10 Mars years, resulting in a leap year with 661 sols.

Even though our calendar doesn't match the Mars year exactly, the date calculations remain precise because they are based on continuous Mars time derived from Julian Date → Mars Sol Date (MSD). This ensures that all conversions from Earth to Mars (and vice versa) remain consistent over time.

When using our custom Mars epoch, the calendar starts on 6 January 2000, 00:00 UTC, which we define as Sol 1, Year 1, Month Orion. This reference point corresponds to Julian Date 2451549.5 and is used as our custom epoch instead of NASA’s reference.

Our Martian calendar has 24 months alternating between 28 and 27 sols (Martian days).
Each pair of months roughly corresponds to an Earth month:

• Orion / Sirius → January
• Vega / Lyra → February
• Cygnus / Altair → March
• Andromeda / Perseus → April
• Aldebaran / Taurus → May
• Castor / Pollux → June
• Regulus / Leo → July
• Spica / Virgo → August
• Antares / Scorpio → September
• Betelgeuse / Canopus → October
• Arcturus / Deneb → November
• Capella / Procyon → December

This pairing is approximate and helps relate Martian months to familiar Earth time periods. Each Mars year contains 668 sols, and a leap sol is added every 10 years to stay in sync.

While a Martian year is approximately 668.6 sols long, our calendar uses a simplified structure of 660 sols per standard year, divided into 24 months alternating between 28 and 27 sols.

This design was chosen for its simplicity and symmetry. The alternating 28/27 pattern is easy to remember and gives the calendar a steady rhythm. Grouped into 12 month-pairs, it also loosely reflects the structure of the 12-month Earth calendar — making it more relatable.

Rather than trying to force the calendar to exactly match the astronomical year, we separate calendar structure from precise timekeeping. Exact Mars time is calculated from the Julian Date → Mars Sol Date (MSD) conversion, which ensures that all conversions between Earth and Mars remain astronomically accurate over time.

To correct for the difference between the 660-sol calendar and the actual Mars year, we introduce a leap sol every 10 Mars years. This leap sol is added at the end of the final month (Procyon) in leap years, helping the calendar stay roughly in sync over decades.