Decimal time units in calendars have a long history.

Egyptian Calendar

The ancient Egyptians divided the year into 36 parts of ten days each, which are now called decans, from the Greek word for ten.  Each decan was associated with a different star the rising of which marked the time of year.  The Egyptian calendar had twelve months, each divided into three decans, plus five extra days at the end of the year, to bring it to 365 days.  Since there was no leap year, the calendar gradually shifted in relation to the seasons over the course of thousands of years.  During the reign of the Roman emperor Augustus, the Alexandrian calendar was introduced, which added a leap day every four years, fixing the new year to August 29 on the Julian calendar.  This calendar continues today in the Coptic and Ethiopian calendars.  It also formed the basis for the Republican calendar during the French Revolution.

Roman Calendar

Calendar of Romulus

The Roman calendar evolved over the centuries into the world's most used calendar. The original calendar was attributed to the legendary founder of Rome, Romulus, in the eight century BCE. It had only ten months, some of which had names based on their order. The seventh through tenth months had names familiar to us today: September, October, November and December, respectively. The fifth and sixth months were named Quintilis and Sextilis, which we now know in English as July and August. The first month was Martius, which is the English March. The winter days between December and March were not counted as part of the calendar. It was probably a lunar calendar, although some sources differ.

Calendar of Numa

The first major reform of the Roman calendar was attributed to King Numa Pompilius around 700 BCE. January and February were added to make the calendar twelve months. Since it was a lunar calendar, it was about ten days shorter than a solar year, so in some years another month was added to February. 

Julian Calendar

In the year 46 BCE, Julius Caesar reformed the Roman calendar so that it was a solar calendar, instead of lunar, so that every year was 365 days long, except that every fourth year had 366 days, the extra day being added to February. After his death two years later, the month of Quintilis was renamed in his honor, which is called July in English. Later, the month of Sextilis was renamed to August, in honor of the first emperor, Augustus. The lengths of the months were adjusted, and this became essentially the same as the calendar that most of the world uses today. It is still used by Eastern Orthodox churches for religious purposes.

Gregorian Calendar

The Julian calendar had one extra day every four years to keep it aligned with the seasons, but over the centuries the seasons slipped. In 1583 CE, Pope Gregory XIV shifted the calendar ten days, so that the vernal equinox would occur around March 21, as it had in the fourth century. He also changed the order of leap years so that every 400 years, three would be skipped, so that there would be only 97 instead of 100, keeping the beginning of spring close to the same date. This is the calendar used by most of the world today.


The Romans divided up their months into three parts, originally corresponding to phases of the moon. The first day of the month, or new moon, was called the Kalends, from which we get the word "calendar". The fifth or seventh day, depending upon the length of the month, was the Nones, corresponding to the first quarter moon, followed nine days later, on the 13th or 15th day, by the Ides, corresponding to the full moon. Caesar was famously murdered on the Ides of March. There was no day named for the last quarter. Rather than counting days from the beginning of the month, dates were denoted by counting down to the next Kalends, Nones or Ides. Thus, the last day of December was called the day before the Kalends of January; the previous day, 30 December, was called day 3 of the Kalends of January, etc. This practice lasted into the Middle Ages.

The early Romans also had an eight-day week, during which one day was designated a market day. During the Empire, the seven-day week used by eastern countries started being used, and was made official by Constantine in 321 CE. It is now universal. In many languages, the days of the week are named for the seven classical planets, which include the sun and moon.

French Republican Calendar

The French Republican Calendar was used in France and countries controlled by France from 1793 to 1805. This calendar was very similar to the ancient Egyptian and Alexandrian calendars, which are still used in Ethiopia and by the Coptic Church. Not only did the Republican Calendar include a decimal week, but also decimal time. Months of the Republican Calendar were divided into three décades, of ten days each. There were twelve 30-day months in a year, plus five or six complementary days at the end. Years began on the autumnal equinox, the Year One being counted retroactively from September 22, 1792, the first year of the Republic. Years were written in Roman numerals, following "l'Année", "An" or "l'An", meaning "year", or "ER" for "Era Républican". Décades of the month were also written in Roman numerals, I, II or III. An example date is "Décade III, Sextidi de Floréal de l'Année VIII de la Révolution", which could also be written as "26 Floréal, an VIII", or "VIII/8/26" for short, which was May 16, 1800 CE. There was a "rural" version, in which each day of the year also had its own name (just as the Catholic Church named days after saints in its calendar) the fifth day of the décade being named after an animal, the tenth day after a farm implement, and the rest after plants or minerals. The ten-day décades were very unpopular due to the division of the workweek and the conflict with Sundays, and stopped being used in Floréal an X (April 1802). The entire calendar was abandoned after Napolean became emperor, at the end of 1805.


The ten days of each décade were called:

Day Name Meaning Day Name Meaning
1 Primidi First Day 6 Sextidi Sixth Day
2 Duodi Second Day 7 Septidi Seventh Day
3 Tridi Third Day 8 Octidi Eight Day
4 Quartidi Fourth Day 9 Nonidi Ninth Day
5 Quintidi Fifth Day 10 Decadi Tenth Day


The months are shown below. Starting dates are approximate, and may fall one or two days later in some years.

Month Starts Name Meaning
1 Sep 22 Vendémiaire Vintage
2 Oct 22 Brumaire Fog
3 Nov 21 Frimaire Frost
Month Starts Name Meaning
4 Dec 21 Nivôse Snow
5 Jan 20 Pluviôse Rain
6 Feb 19 Ventôse Wind
Month Starts Name Meaning
7 Mar 21 Germinal Germination
8 Apr 20 Floréal Flowers
9 May 20 Prairial Prairies
Month Starts Name Meaning
10 Jun 19 Messidor Harvest
11 Jul 19 Thermidor Heat
12 Aug 18 Fructidor Fruits

The five or six days (depending on leap years) added to the end of the year were called:

Day Name Meaning
1 Jour de la vertu Virtue Day
2 Jour du génie Genius Day
3 Jour du travail Work Day
4 Jour de l'opinion Opinion Day
5 Jour des récompenses Rewards Day
6 Jour de la révolution Revolution Day

Leap Years

Leap years, called sextile because they contained a sixth complementary day, occurred whenever two consecutive autumnal equinoxes happened to fall 366 days apart, as observed in Paris, which happened in the years III, VII and XI. A period of four years, at the end of which the addition of one day was necessary, was called a franciade, However, had the calendar continued in use, there would have been five years between the leap years XV and XX. There was also a problem that when the equinox occurred close to midnight, the margin for error made it impossible to predict whether it would fall on the day before or after midnight.
In the year III Gilbert Romme proposed rules similar to those of the Gregorian calendar, so that years divisible by 4 would be leap years, unless they were divisible by 100 and not divisible by 400. Also, the year 4000 would not be a leap year. However, he was sentenced to the guillotine and committed suicide shortly after, and the original equinox rule was followed, instead, with the first leap day occurring three months after his death. This method has the benefit that Republican years start on the same day in the Gregorian calendar for long periods; for instance, all years start on September 22 between 1993 and 2092.
Some concordances printed in France after 1805 continued having every four years after year 11 be leap years, i.e. 15, 19, 23, 27, 31, etc. Some in France today also calculate leap years this way, applying Romme's reformed rules to the year previous to what it would have been, so that the extra day is inserted immediately before most years divisible by four instead of added to the end. Thus, every four years since year 3 was a leap year, except years 99 and 199.
There are some who use a rule that from the year 20 on, years divisible by 4 are leap years, unless they are also divisible by 128. There seems to be no historical precedent for this rule.

Converting to Gregorian Calendar

The following table displays dates according to the Common Era for the first day of several years of the Republican Era, according to four methods of determining leap years:
  • Original rule: years start on equinox
  • Romme's revised rule: leap day added at end of years divisible by four, except centuries
  • Continuous rule: leap day inserted before years divisible by four, except centuries
  • 128-year rule: leap day added at end of years divisible by four, unless divisible by 128
ER CE Equinox Romme Continuous 128-Year
CCXXIII (223) 2014 Sept. 23* Sept. 22 Sept. 22 Sept. 23
CCXXIV (224) 2015 Sept. 23 Sept. 22 Sept. 23* Sept. 23
CCXXV (225) 2016 Sept. 22 Sept. 22*Sept. 22 Sept. 23*
CCXXVI (226) 2017 Sept. 22 Sept. 22 Sept. 22 Sept. 23
CCXXVII (227) 2018 Sept. 23* Sept. 22 Sept. 22 Sept. 23
CCXXVIII (228) 2019 Sept. 23 Sept. 22 Sept. 23* Sept. 23
CCXXIX (229) 2020 Sept. 22 Sept. 22*Sept. 22 Sept. 23*
CCXXX (230) 2021 Sept. 22 Sept. 22 Sept. 22 Sept. 23
* Follows leap day at end of previous year