Tranquility Calendar

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The Tranquility Calendar is a solar calendar proposal for calendar reform proposed by Jeff Siggins in 1989.[1][2] It is a derivative of the International Fixed Calendar as well as the earlier Positivist Calendar published in 1849 by French philosopher Auguste Comte (1798–1857), providing for a year of 13 months of 28 days each, with one day at the end of each year belonging to no month or week, and a leap day approximately every 4 years.


The calendar's center date in history is called Moon Landing Day. The actual center point in time is the exact moment the word tranquility is mentioned in this somewhat famous quote — "Houston, Tranquility Base Here. The Eagle Has Landed". Moon Landing Day has 20 hours, 18 minutes, and 1.2 seconds Before Tranquility and 3 hours, 41 minutes, 58.8 seconds After Tranquility.

The calendar year has 13 months each with 28 days (divided in exactly 4 weeks) plus an extra day at the end of the year not belonging to any month (and so 365 days). The months are named after famous scientists: Archimedes, Brahe, Copernicus, Darwin, Einstein, Faraday, Galileo, Hippocrates, Imhotep, Jung, Kepler, Lavoisier, and Mendel. The 365th day is called Armstrong Day; the leap day is called Aldrin Day.

Days that do not belong to a month are deemed to be outside the week and always occur between a day deemed Thursday and a day deemed to be Friday. The first day of each year, Archimedes 1, is deemed a Friday and every subsequent day that belongs to a month is deemed to be in the conventional 7-day week.

The year starting the day after Moon Landing Day, and lasting until Armstrong Day, is designated 1 After Tranquility, or 1 AT, with subsequent years numbered in order; the year lasting from July 2013 until July 2014 is 45 AT, and the year lasting from July 2014 until July 2015 is 46 AT. The year ending the day before Moon Landing Day, and starting on the previous Armstrong Day, is 1 Before Tranquility, or 1 BT.

While Moon Landing Day's definition roughly correlates the calendar to Greenwich Mean Time, as there are no leap seconds, it more closely approximates TAI than UTC.

The leap year system matches the Gregorian Calendar; thus, the 400-year cycle of leap years starts on 31 AT - every four years is a leap year, except for every hundredth year which is not, except for every four hundredth year which is.

Because each month consists of exactly four weeks, the first day of each month and every seventh day after that for the rest of the month is deemed to be a Friday, the second day of each month and every seventh day after that for the rest of the month is deemed to be a Saturday, and so on. Therefore, each month begins on a Friday and ends on a Thursday.

This causes all months to look like this:

Fri Sat Sun Mon Tue Wed Thu
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28

The 13 months and extra days occur on the following Gregorian dates:

Month Starts Ends
Archimedes July 21 August 17
Brahe August 18 September 14
Copernicus September 15 October 12
Darwin October 13 November 9
Einstein November 10 December 7
Faraday December 8 January 4
Galileo January 5 February 1
Hippocrates February 2 March 1
Aldrin Day February 29
Imhotep March 2 March 29
Jung March 30 April 26
Kepler April 27 May 24
Lavoisier May 25 June 21
Mendel June 22 July 19
Armstrong Day July 20


Several advantages do exist on this calendar, mainly related to its organization. When compared to the Gregorian, it is clear that this calendar is much simpler and practical:

  • every year has exactly 52 weeks divided in 13 months;
  • each month has exactly 28 days divided in 4 weeks;
  • every month/year's day has the same week day (i.e. n'th month/year day is always the m'th week's day, where m is the remainder of n/7).

The Tranquility calendar is perennial, keeping the same days per year, and the same days per week, which are two advantages that ease possible change. On this proposal, the number of national holidays that do not fall on weekends are no longer year dependent. This no longer causes certain years to have more workdays than others.


  • Thirteen, being a prime number, cannot be evenly divided, putting all activities currently done on a quarterly basis out of alignment with the months. (However, with the weeks not being disrupted this would be handled with relative ease: the first quarter would end with the first week of the fourth month; the second quarter with the second week of the seventh month; and so on.)
  • Several religious groups oppose any interruption of the seven-weekday sequence.[3]


The collaborative science fiction writing project, Orion's Arm uses the Tranquility Calendar.[4]

See also


  1. Siggins, Jeff (July 1989). "Lunar Timekeeper: A Special Lunar Calendar for the Space Age". OMNI Magazine 11(10):96. Archived from the original on March 9, 2015. Retrieved March 9, 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. "OMNI cover July 1989 and table of contents". OMNI. July 1989. Archived from the original on May 10, 2012. Retrieved March 9, 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  3. Joseph Herman Hertz, Calendar Reform
  4. "The Tranquility Calendar". Orion's Arm. Archived from the original on March 6, 2015. Retrieved March 6, 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>