The Chronicles of Local Space

Martian Calendar

The Martian colonies use a perpetual calendar with all months beginning on Monday. Years may be 672-day complete years with all 24 months having 28 days, or 665-day deficient years with the last month of the 24-month year having only 21 days. An intercalation cycle takes 76 years, made up of 39 complete years and 37 deficient years, for an average year length of 95³⁹⁄₇₆ weeks, or about 668.5921 Martian days. The calendar follows the seasons with good accuracy, which is necessary on such a harsh world.

Standard Months

The Martian Business Calendar has 24 months in a year, and most months are arranged as follows.

Each week of the month also has a week number, in a similar manner to the standard weeks as specified in ISO 8601. This makes business planning easier, because one can determine the number of weeks between two dates by subtracting one week number from the other, rather than counting by hand. In addition to the month and day of the month, Martian dates can be specified by week number and day of the week because a particular date can always be converted to the same week number and day of the week. For example, the first day of the first month is always Monday in week 1.

Deficient Months

The only exception to the 28-day month is the 24th month in short years. It has only three weeks, and appears as follows.

In short years, anniversaries of events that occured during an intercalary week (days 22 to 28 in the 24th month) are instead observed on the corresponding day of the week during the last week of the year (days 15 to 21 of the 24th month).

This system of mapping dates is a little unusual, but makes sense when one considers that the days in the last week of the year can also be expressed as occurring a fixed number of days before the start of the following year. It is similar to the system of dates employed in the old Roman calendar. The old Roman calendar did not give each day a separate number as we do today, but instead gave three days of each month special names (Kalendae, Nōnae and Idūs), and then expressed the intervening dates as the number of days until the next named day. Kalendae were the first day of the month, Nōnae were the 5th or 7th day of the month, and Idūs were the 13th or 15th day of the month. For example, the date corresponding to our modern date of April 28 would be called “ANTE DIEM IV KALENDAS MAIVM” (A.D. IV KAL. MAI.), or “four days before the first of May”. The Romans counted four days not three because the Romans counted both days to determine the interval.

Month Names

The months are numbered, not named; the months are simply called 1st Month, 2nd Month and so on. Such a system makes the month names easy to remember. This convention for month names is similar to the names of the months in the Gregorian calendar as used in China, Japan and other similar places. In China and Japan, the months are named with the number of the month combined with the character for “moon” (1月, 2月, 3月 and so on. It is also the origin of the Latin names for the months September (seventh month), October (eighth month), November (ninth month) and December (tenth month), as well as the old Latin names for the months of July (Quintilis, fifth month) and August (Sextilis, sixth month).

Epoch

The epoch of the Martian calendar is January 1, Year 1 in the Gregorian Proleptic Calendar.

Martian Calendar

The intercalary week is indicated in red.

Further Reading

A more detailed description of the Martian Calendar may be found here.

This document was last modified 2010-09-12 11:34:35 UTC.
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