How long is a year on Mars?

Just about any astronomy book will tell you that the orbital period of Mars is 686.97 days. But this measurement is in 24-hour Earth days, not Martian solar days, which are almost forty minutes longer. If you lived on Mars, you would count approximately 668.59 Martian days. For a discussion on the various astronomical years, see Michael Allison's "What is a 'Year' (on Earth or Mars)?".

Table of Calendars

How many days should be added in a leap year?

General

1 Day

Pro

This is the simplest solution. The leap year is only one day longer than the non-leap year. This minimizes "the anniversary problem."

Con

Source Documents:

	Arih Martian Calendar Igor Arih
	Arihrov Marsovski Koledar Igor Arih
	Seasons on Mars Frank N. Bauregger
	A Thought for the Future William H. Becker
	The Darian Defrost Calendar Frans Blok
	Mars Calendar Blort
	Simplest Possible Mars Calendar Used in the Novel As It Is On Mars Thomas W. Cronin
	Time Keeping on Mars Alan Dechert
	600-Year Martian Calendar Alan Dechert
	Excerpts from Martian Rainbow Robert L. Forward
	The Darian System Thomas Gangale
	Excerpts from Across the Zodiac Percy Greg
	The Millennium Mars Calendar James M. Graham and Kandis Elliot
	The Fortieth of July Pierre Hallet
	An Easily Understood Calendar and Time System for Mars Leon G. Heron
	The Thothian Wheel Fred Hightower
	Rationale for the Martian Calendar's Structure Bill Hollon
	Further Thoughts on Vophicism Mark Knoke
	Towards a Better Martian Calendar Mark Knoke
	A Four Season Split-year Calendar for Mars Peter Kokh
	The "Mars Pulse" Calendar Peter Kokh
	Calendar for Mars Mike Kretsch
	SCDTL Calendar and Clock for Mars Lance Latham
	It's About Time I. M. Levitt
	Mars Clock and Calendar I. M. Levitt
	Excerpts from The Greening of Mars James Lovelock and Michael Allaby
	The Utopian Calendar Mars Time Group
	The Areosynchronous Calendar Shaun Moss
	The Kepler Calendar Shaun Moss
	Martian Dreams Time System Linda Naughton and Robert O'Meara
	The QS Calendar Jelmer Oosthoek
	Elemental Martian Calendars Terry Phelan
	Excerpts from Red Mars Kim Stanley Robinson
	Salvas Calendar System for Mars Keith Salvas
	Martian Calendar Anton Sherwood
	Martian Timekeeping Anders Ström
	A Calendar for Mars Josef Šurán
	A Mars Calendar Richard Weidner
	Options for Martian Timekeeping William Woods

1 or 2 Days (two types)

Pro

Con

Having two types of leap years means that there would by years of three different lengths (including the non-leap year), which would be more confusing than having a non-leap year and just one type of leap year.

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having two dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Martian Calendar Roger E. Kuiper
	Calendar for Mars Robert S. Richardson

2 Days

Pro

This is a simple solution.

Con

This solution is somewhat less elegant than having the leap year be only one day longer than the non-leap year.

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having as many as three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Calendar for Mars Robert S. Richardson

2 or 3 Days (two types)

Pro

Con

Having two types of leap years means that there would by years of three different lengths (including the non-leap year), which would be more confusing than having a non-leap year and just one type of leap year.

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having as many as three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

Martian Clock and Calendar Mickey D. Schmidt

3 Days

Pro

This is a simple solution.

Con

This solution is somewhat less elegant than having the leap year be only one day longer than the non-leap year.

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Time Measures on Mars Robert G. Aitken
	A Mars Proleptic Calendar and Sol-Date Timing Reference Michael Allison
	Space Studies 200 Group Project Ryan Kramer et al.
	Calendar for Mars Robert S. Richardson
	Calendario en Marte Miguel Angel Serra Martín
	Mars Calendar Miguel Angel Serra Martín

Entire 7-day Week

Pro

This solution accommodates a perpetual calendar, i.e., one in which every year begins on the same day of the week, without the need for intercalary days (days that are not counted as part of the week).

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having seven dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Martian Timekeeping Geoffrey Briggs and Howard Houben
	A Calendar for Mars Josef Šurán

Entire 10-day Week

Pro

This solution accommodates a perpetual calendar, i.e., one in which every year begins on the same day of the week, without the need for intercalary days (days that are not counted as part of the week).

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having ten dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	The Martian Calendar M. Vertregt
	Martian Year Revisited M. Vertregt and Francis Graham

Entire 29-day or 30-day Month

Pro

This solution attempts to keep Martian months in sychronization with months on the Gregorian calendar on Earth.

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having 30 dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

Time on Mars John J. G. Savard

What should be the basic leap year scheme?

General

For 1 leap day per year:

General

Add one day as determined by observation

Pro

Con

This system would have not have a simple, repeatable pattern, and would be very difficult to put into practice. Future dates could not be predicted with absolute certainty.

Source Documents:

	Rationale for the Martian Calendar's Structure Bill Hollon
	The QS Calendar Jelmer Oosthoek

Add one day every 2 out of 3 years, except every 15 years

This system results in an average calendar year of 668.60 days over a 15-year cycle.

Pro

Con

Try this test: is the year 1969 a leap year or a non-leap year?

Add one day every 3 out of 5 years

This system results in an average calendar year of 668.60 days over a 5-year cycle.

Pro

Con

Try this test: is the year 1969 a leap year or a non-leap year?

Source Documents:

	Arih Martian Calendar Igor Arih
	Arihrov Marsovski Koledar Igor Arih
	Excerpts from Martian Rainbow Robert L. Forward
	It's About Time I. M. Levitt
	Mars Clock and Calendar I. M. Levitt
	Martian Calendar Anton Sherwood

Add one day every 15/8 Earth years

Pro

Con

It sounds simple enough. If fact, it's an oversimplifacation that results in an error of more than two days in only a year. Multiplying a common (not tropical) Earth year (365) by 15/8, then dividing by 1.027491 to account for the longer Martian day, results in a Martian year of 666.06 days. The correct figure is 668.5921 days.

Source Documents:

	Mars Time Elysium Mars
	A Critique of Robert Zubrin's Martian Calendar Thomas Gangale and Marilyn Dudley-Rowley
	An Explanation of Zubrin's Mars Calendar Paul J. Thomas
	A Calendar for Mars Robert Zubrin
	A Calendar for the Planet Mars Robert Zubrin

Add one day in odd years + decennial years

This system results in an average calendar year of 668.60 days over a 10-year cycle.

Pro

This system is one of the easiest to use in everyday life. All odd-numbered years have 669 days (leap years), which is an odd number. All even-numbered years are 668 days (non-leap years), which is an even number, except that all years divisibile by ten (ending in a "0") are 669 days (leap years). Try this test: is the year 1969 a leap year or a non-leap year? Now, try the same test on all of the other proposed systems.

Con

Source Documents:

	Seasons on Mars Frank N. Bauregger
	A Thought for the Future William H. Becker
	The Darian Defrost Calendar Frans Blok
	Mars Calendar Blort
	Simplest Possible Mars Calendar Used in the Novel As It Is On Mars Thomas W. Cronin
	Time Keeping on Mars Alan Dechert
	600-Year Martian Calendar Alan Dechert
	The Darian System Thomas Gangale
	An Easily Understood Calendar and Time System for Mars Leon G. Heron
	Further Thoughts on Vophicism Mark Knoke
	Towards a Better Martian Calendar Mark Knoke
	SCDTL Calendar and Clock for Mars Lance Latham
	The Utopian Calendar Mars Time Group
	The Areosynchronous Calendar Shaun Moss
	The Kepler Calendar Shaun Moss
	Elemental Martian Calendars Terry Phelan
	A Calendar for Mars Josef Šurán
	Options for Martian Timekeeping William Woods

Subtract one day every 3 years + decennial years

Pro

Con

This system is based on the erroneous premise that there are 669.6 Martian solar days in a Martian year. Actually, this is the number of Martian sidereal days in a Martian year!

Source Documents:

Salvas Calendar System for Mars Keith Salvas

Subtract one day every 51 years

Pro

Con

This system is based on the erroneous premise that there are 686.98 Martian days in a Martian year. Actually, this is the number of Earth days in a Martian year!

Source Documents:

Excerpts from The Greening of Mars James Lovelock and Michael Allaby

Atomic cycles, minor cycles, and major cycles

This system results in an average calendar year of 668.5903 days over a 310-year cycle.

The smallest atomic cycle is a group of five years. The first, third, and fifth year are leap years. The next smallest atomic cycle is a group of seven years where the first, third, fifth, and seventh year are leap years.

The minor cycles are composed of four atomic cycles. Four of the minor cycles are composed of three five-year cycles and one seven-year cycle, in the permutations of that group (ie 7555,5755,5575,5557). The fifth minor cycle is composed of two seven year cycles and two five year cycles (7557). Thus the minor cycles are all 22 years except for the fifth cycle which is 24 years. The minor cycles are enumerated by 1: 7555, 2: 5755, 3:5575, 4: 5557, and 5: 7557.

The major cycle is 310 years long, consisting of the sequence of minor cycles, 5,4,4,3,4,3,3,2,3,2,2,2,2,1.

Pro

Con

There are 3343 days in a five-year atomic cycle and and 4680 days in a seven-year atomic cycle. So minor cycles 1 through 4 have 14709 days (three five-year atomic cycles and one seven-year atomic cycle) and minor cycle 5 has 16046 days (two five-year atomic cycles and two seven-year atomic cycles). There are 13 of the minor cycle 1 through 4 types and one minor cycle 5 in a 310-year major cycle, for a total of 207263 days. After all of that, the author states that his 310-year series does not repeat. "There is no reason to believe that the series holds outside the span of 1609 to 2910 CE Gregorian Calendar."

The question that cuts the Gordian Knot is this: Is the year 1969 a leap year or a non-leap year?

Source Documents:

A Mars Calendar Richard Weidner

For 1 or 2 leap days per year (two types):

General

Add days as a function of the position of Earth.

Pro

Add days as a function of the position of Earth to create a celebration of the mother planet.

Con

Add one day in even years + one more day in decennial years

This system results in an average calendar year of 668.60 days over a 10-year cycle.

Pro

Con

The major detraction of this system is the complication of having two different types of leap years.

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Calendar for Mars Robert S. Richardson

Add one day every 38 years + one more day every 590 years

This system results in an average calendar year of 667.0526 days over a 590-year cycle.

Pro

Con

This system is based on the erroneous premise that there are 667 Martian days in a Martian year. It has the added detraction of the complication of having two different types of leap years.

Source Documents:

Martian Calendar Roger E. Kuiper

For 2 leap days per year:

General

Subtract two days every 5th year

This system results in an average calendar year of 668.60 days over a 5-year cycle.

Pro

This system is the easiest to use in everyday life. All years divisible by five (ending in either "5" or "0") are 667 days. All other years are 669 days. Try this test: is the year 1969 a leap year or a non-leap year? Now, try the same test on all of the other proposed systems.

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Calendar for Mars Robert S. Richardson

For 2 or 3 leap days per year (two types):

General

Add two days every 5th year + one more day every 300th year

Years divisible by five contain 670 days, except that years divisible by 300 contain 671 days. All other years contain 668 days. This system results in an average calendar year of 668.4086 days over a 300-year cycle.

Pro

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

This system has the additional complication of having two different types of leap years.

Finally, this leap year scheme is inaccurate. It is off by an entire day after only five years.

Source Documents:

Martian Clock and Calendar Mickey D. Schmidt

For 3 leap days per year:

General

Add three days every 5th year

This system results in an average calendar year of 668.60 days over a 5-year cycle.

Pro

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Time Measures on Mars Robert G. Aitken
	A Mars Proleptic Calendar and Sol-Date Timing Reference Michael Allison
	Calendar for Mars Robert S. Richardson

Add three days every 5th year

This system results in an average calendar year of 669.60 days over a 5-year cycle.

Pro

Con

This system is based on the number of sidereal days in a year rather than the munber of solar days in a year. This error results in a calendar year that is one day longer than the actual year.

Additionally, the principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having three dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Space Studies 200 Group Project Ryan Kramer et al.
	Calendario en Marte Miguel Angel Serra Martín
	Mars Calendar Miguel Angel Serra Martín

For an entire 7-day leap week per year:

General

Add seven days in even years + every 35th odd year

This system results in an average calendar year of 668.60 days over a 70-year cycle.

Pro

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having seven dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

A Calendar for Mars Josef Šurán

Add seven days in even years + every 50th odd year

This system results in an average calendar year of 668.57 days over a 100-year cycle.

Pro

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having seven dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Additionally, this intercalation system is off by two days within the defined cycle of 100 years.

Source Documents:

Martian Timekeeping Geoffrey Briggs and Howard Houben

For an entire 10-day leap week per year:

General

Add ten days every 7th year + every 50th year

This system results in an average calendar year of 669.57 days over a 7-year cycle. After seven cycles (49 years), a 670-day year is inserted, resulting in an average calendar year of 668.60 days over a 50-year cycle.

Pro

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having ten dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	The Martian Calendar M. Vertregt
	Martian Year Revisited M. Vertregt and Francis Graham

For an entire 29-day or 30-day embolismic month per year:

General

Add a month every four years out of seven years

This system results in an average calendar year of 669.5714 days over a 7-year cycle. After seven cycles (49 years), a 670-day year is inserted, resulting in an average calendar year of 668.5918 days over a 50-year cycle.

Pro

This solution attempts to keep Martian months in sychronization with months on the Gregorian calendar on Earth.

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having 30 dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

Time on Mars John J. G. Savard

When should the leap days occur?

General

Beginning of the year

Pro

Con

If the leap day is an intercalary day, this makes sense. Otherwise, the leap day should be at the end of the year.

End of 1st month

Pro

Con

The leap day should be at the end of the year.

Source Documents:

	Calendar for Mars Mike Kretsch
	The QS Calendar Jelmer Oosthoek
	Salvas Calendar System for Mars Keith Salvas

End of 1st, 2nd, and 3rd months

Pro

Con

The leap day should be at the end of the year.

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having two dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

Space Studies 200 Group Project Ryan Kramer et al.

End of 2nd month (of 12)

Pro

This mimics "February 29th" on Earth's Gregorian calendar.

Con

This idea blindly follows a historical precedent which no longer has a logical reason behind it. February was once the last month of the year, and the leap day should be at the end of the year.

Source Documents:

	Seasons on Mars Frank N. Bauregger
	Mars Calendar Blort
	The Darian System Thomas Gangale
	Martian Timekeeping Anders Ström

End of 2nd month (of 22)

Pro

This mimics "February 29th" on Earth's Gregorian calendar.

Con

This idea blindly follows a historical precedent which no longer has a logical reason behind it. February was once the last month of the year, and the leap day should be at the end of the year.

Source Documents:

A Mars Proleptic Calendar and Sol-Date Timing Reference Michael Allison

End of 3rd month (of 12)

Pro

This mimics "February 29th" on Earth's Gregorian calendar.

Con

This idea blindly follows a historical precedent which no longer has a logical reason behind it. February was once the last month of the year, and the leap day should be at the end of the year.

Source Documents:

An Easily Understood Calendar and Time System for Mars Leon G. Heron

End of 4th month (of 24)

Pro

This mimics "February 29th" on Earth's Gregorian calendar.

Con

This idea blindly follows a historical precedent which no longer has a logical reason behind it. February was once the last month of the year, and the leap day should be at the end of the year.

End of 5th month (of 12)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

A Four Season Split-year Calendar for Mars Peter Kokh

End of 8th month (of 12)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

Martian Timekeeping Geoffrey Briggs and Howard Houben

End of 11th month (of 16)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

Towards a Better Martian Calendar Mark Knoke

End of 12th, 18th and 24th months (of 24)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

Martian Clock and Calendar Mickey D. Schmidt

End of 13th month (of 24)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

Martian Calendar Anton Sherwood

End of 19th month (of 24)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

Martian Calendar Anton Sherwood

End of 22nd month (of 32)

Pro

Con

The leap day should be at the end of the year.

Source Documents:

Further Thoughts on Vophicism Mark Knoke

End of the year

Pro

This is the most logical place to insert the leap day. On Earth, March (honoring Mars) was originally the first month on the Roman calendar (September through December were the seventh through the tenth months, as their names imply), and February was the last month. In this context, it made sense for the leap day to be at the end of February. It was the last day of the year.

Con

Source Documents:

	The Darian Defrost Calendar Frans Blok
	The Rotterdam Month and Weekday Naming System Frans Blok
	Waar blijft de tijd op Mars? Frans Blok
	Simplest Possible Mars Calendar Used in the Novel As It Is On Mars Thomas W. Cronin
	The Darian System Thomas Gangale
	The Millennium Mars Calendar James M. Graham and Kandis Elliot
	Rationale for the Martian Calendar's Structure Bill Hollon
	A Four Season Split-year Calendar for Mars Peter Kokh
	The "Mars Pulse" Calendar Peter Kokh
	A New Calendar and Clock for the Planet Mars Manfred Krutein
	It's About Time I. M. Levitt
	Mars Clock and Calendar I. M. Levitt
	The Utopian Calendar Mars Time Group
	The Areosynchronous Calendar Shaun Moss
	The Kepler Calendar Shaun Moss
	Martian Dreams Time System Linda Naughton and Robert O'Meara
	Elemental Martian Calendars Terry Phelan
	Martian Calendar Anton Sherwood

Mid-year

Pro

Con

The leap day should be at the end of the year.

Source Documents:

	A Thought for the Future William H. Becker
	A Four Season Split-year Calendar for Mars Peter Kokh
	The "Mars Pulse" Calendar Peter Kokh
	Time on Mars John J. G. Savard
	Martian Calendar Anton Sherwood
	A Calendar for Mars Josef Šurán

Mid-year and end of the year

Pro

For leap yeap schemes that require two leap days in a leap year, this is the logical place to put them.

Con

The principal objection to any scheme involving more than one leap day is "the anniversary problem." On Earth, people born on 29 February can truly only celebrate a birthday once every four years. Imagine having two dates that may or may not occur in a given year. And of course, the problem is not restricted to birthdays, but to any anniversary that might be observed. Suppose Martian independence is declared on a date that doesn't occur every year!

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Calendar for Mars Robert S. Richardson

Vernal equinox and autumnal equinox

Pro

Con

The variable day occurs at two different times of the year, depending on the year: either on the vernal equinox or on the autumnal equinox. This is unnecessarily complicated.

The leap day should be at the end of the year.

Source Documents:

Excerpts from Martian Rainbow Robert L. Forward

What period of time should contain an integral number of weeks in order to create a perpetual calendar?

General

Originally, the Roman calendar divided the months into three integral periods: the Nones, the Ides, and the Calends. When the seven-day week was incorporated into the Julian calendar in the 4th century, there was no attempt to integrate this new period of time into either the month or the year; rather, the week was allowed to "float" through the months and years. The idea of "fixing" the week is at the core of all proposals to reform Earth's Gregorian calendar. On a perpetual calendar, days of the week and numerical dates repeat over a defined cycle.

1 Month

Pro

In this system, you will always know that the 10th of the month is a Tuesday, for instance. This system simplifies accounting and maximizes economic efficiency, which will be essential to the success of Martian colonies.

Con

If the first day of the month is always Sunday, the 13th is always Friday. Lucky us.

Source Documents:

	A Thought for the Future William H. Becker
	The Darian Defrost Calendar Frans Blok
	The Rotterdam Month and Weekday Naming System Frans Blok
	Waar blijft de tijd op Mars? Frans Blok
	Martian Timekeeping Geoffrey Briggs and Howard Houben
	The Importance of Calendar Reform to the Business World George Eastman
	The Darian System Thomas Gangale
	The Thothian Wheel Fred Hightower
	Rationale for the Martian Calendar's Structure Bill Hollon
	History of the 13-Month Calendar Journal of Calendar Reform
	Calendar for Mars Mike Kretsch
	It's About Time I. M. Levitt
	Mars Clock and Calendar I. M. Levitt
	The Utopian Calendar Mars Time Group
	The Areosynchronous Calendar Shaun Moss
	The Kepler Calendar Shaun Moss
	Martian Dreams Time System Linda Naughton and Robert O'Meara
	Elemental Martian Calendars Terry Phelan
	Martian Clock and Calendar Mickey D. Schmidt
	A Calendar for Mars Josef Šurán

1 Year

Pro

This idea is at the core of all proposals to reform Earth's Gregorian calendar. Every year is the same, except for the leap day. For a specific month, the 12th day would always occur on the same day of the week. If you were born on a Saturday, your birthday would be on Saturday every year.

Con

What day of the week is the 10th of next the month?

If we are going to integrate the week into the Martian calendar, why not go all the way? Let's integrate it into the month, rather than just into the year.

Source Documents:

	Calendrier Martien du GCM François Forget et al.
	Excerpts from Across the Zodiac Percy Greg
	A Four Season Split-year Calendar for Mars Peter Kokh
	The "Mars Pulse" Calendar Peter Kokh
	A New Calendar and Clock for the Planet Mars Manfred Krutein
	A New Martian Timekeeping and Mapping System Manfred Krutein
	The World Calendar Rick McCarty
	The QS Calendar Jelmer Oosthoek
	The Martian Calendar M. Vertregt
	Martian Year Revisited M. Vertregt and Francis Graham

2 Years

Pro

This system effects a perpetual calendar using only one day that fall outside of the week--the leap day.

Con

What day of the week is the 10th of next the month?

If we are going to integrate the week into the Martian calendar, why not go all the way? Let's integrate it into the month, rather than just into a biennial cycle.

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Calendar for Mars Robert S. Richardson

None

Pro

All perpetual calendars require some deviation from the standard week, whether that standard is seven days or some other number. Periodically, either a day must be inserted that does not count as a day of the week, or the week must be shortened by a day from time to time.

We've been getting along on Earth without a perpetual calendar. Why do we need one on Mars?

Con

What day of the week is the 10th of next the month?

The cause of calendar reform has stagnated on Earth because of cultural inertia. On Mars, we will have the opportunity to make a fresh start on many things, and this is one of them.

Source Documents:

	A Mars Proleptic Calendar and Sol-Date Timing Reference Michael Allison
	Arih Martian Calendar Igor Arih
	Arihrov Marsovski Koledar Igor Arih
	Seasons on Mars Frank N. Bauregger
	Mars Calendar Blort
	Simplest Possible Mars Calendar Used in the Novel As It Is On Mars Thomas W. Cronin
	Time Keeping on Mars Alan Dechert
	600-Year Martian Calendar Alan Dechert
	Excerpts from Martian Rainbow Robert L. Forward
	The Millennium Mars Calendar James M. Graham and Kandis Elliot
	An Easily Understood Calendar and Time System for Mars Leon G. Heron
	Martian Calendar Roger E. Kuiper
	Excerpts from The Greening of Mars James Lovelock and Michael Allaby
	Elemental Martian Calendars Terry Phelan
	Salvas Calendar System for Mars Keith Salvas
	Time on Mars John J. G. Savard

For the purpose of devising a perpetual calendar, which of the following deviations is most desirable?

General

Shorten the week by one day, three to four times per year

Pro

Spreading the deleted days of the week evenly throughout the year results in the most balanced calendar possible, with each quarter having nearly the same number of days.

Con

Source Documents:

	The Darian Defrost Calendar Frans Blok
	The Rotterdam Month and Weekday Naming System Frans Blok
	Waar blijft de tijd op Mars? Frans Blok
	The Darian System Thomas Gangale
	Martian Dreams Time System Linda Naughton and Robert O'Meara
	Calendar for Mars Mike Kretsch
	It's About Time I. M. Levitt
	Mars Clock and Calendar I. M. Levitt
	The Utopian Calendar Mars Time Group
	Martian Clock and Calendar Mickey D. Schmidt
	A Calendar for Mars Josef Šurán

Shorten the week by three or four days at the end of the year

Pro

Con

Shortening the weeks by three or four days at the end of the year seriously disrupts the weekly rhythm of society. It also results in a lopsided calendar, with three quarters lasting 168 days and the final quarter containing only 164 or 165 days.

Source Documents:

	Excerpts from The Greening of Mars James Lovelock and Michael Allaby
	Elemental Martian Calendars Terry Phelan

Add a day that does not fall within the weekly scheme (a holiday), several times per year

Pro

Con

Source Documents:

	A Thought for the Future William H. Becker
	Excerpts from Martian Rainbow Robert L. Forward
	The Thothian Wheel Fred Hightower

Add a day that does not fall within the weekly scheme (a holiday) in leap years

Pro

Adding an intercalary day approximately every two years results in the least deviation from the seven-day week.

Con

This scheme supports a system in which the weekly and monthly cycles repeat after two years. This is not nearly as attractive as systems in which each month begins on the say day of the week.

Source Documents:

	Time Measures on Mars Robert G. Aitken
	Calendar for Mars Robert S. Richardson

Add 3 or 4 days that do not fall within the weekly scheme (holidays)

Pro

Con

Adding 3 or 4 days at the end of the year that do not fall within the weekly scheme seriously disrupts the weekly rhythm of society. It simply is not credible that the entire society would take these days as holidays, during which all economic activity would come to a halt.

Source Documents:

Elemental Martian Calendars Terry Phelan

Add 8 or 9 days that do not fall within the weekly scheme (holidays)

Pro

Con

Adding 8 or 9 days at the end of the year that do not fall within the weekly scheme seriously disrupts the weekly rhythm of society. It simply is not credible that the entire society would take these days as holidays, during which all economic activity would come to a halt.

Source Documents:

Elemental Martian Calendars Terry Phelan

Add 10 or 11 days that do not fall within the weekly scheme (holidays)

Pro

Con

Adding 10 or 11 days at the end of the year that do not fall within the weekly scheme seriously disrupts the weekly rhythm of society. It simply is not credible that the entire society would take these days as holidays, during which all economic activity would come to a halt.

Source Documents:

The "Mars Pulse" Calendar Peter Kokh

Add an entire week in leap years

Pro

Adding an entire week at the end of the year in leap years maintains the weekly rhythm of society without disruption.

Con

Adding an entire week at the end of the year in leap years results in a lopsided calendar. For calendars with seven-day weeks, three quarters last 168 days and the final quarter contains 175 days. For a calendar with ten-day weeks, it is even worse, with one or two quarters lasting 160 days, and one or two quarters lasting 170 days.

Source Documents:

	Martian Timekeeping Geoffrey Briggs and Howard Houben
	A Calendar for Mars Josef Šurán
	The Martian Calendar M. Vertregt
	Martian Year Revisited M. Vertregt and Francis Graham

The calendar should not contain weeks or should not be perpetual

Pro

All perpetual calendars require some deviation from the standard week, whether that standard is seven days or some other number. Periodically, either a day must be inserted that does not count as a day of the week, or the week must be shortened by a day from time to time.

We've been getting along on Earth without a perpetual calendar. Why do we need one on Mars?

Con

What day of the week is the 10th of next the month?

The cause of calendar reform has stagnated on Earth because of cultural inertia. On Mars, we will have the opportunity to make a fresh start on many things, and this is one of them.

Source Documents:

	A Mars Proleptic Calendar and Sol-Date Timing Reference Michael Allison
	Arih Martian Calendar Igor Arih
	Arihrov Marsovski Koledar Igor Arih
	Seasons on Mars Frank N. Bauregger
	Mars Calendar Blort
	Simplest Possible Mars Calendar Used in the Novel As It Is On Mars Thomas W. Cronin
	Time Keeping on Mars Alan Dechert
	600-Year Martian Calendar Alan Dechert
	The Millennium Mars Calendar James M. Graham and Kandis Elliot
	An Easily Understood Calendar and Time System for Mars Leon G. Heron
	Martian Calendar Roger E. Kuiper
	Salvas Calendar System for Mars Keith Salvas
	Time on Mars John J. G. Savard