French Revolutionary Calendar

06bourb.jpg (12105 bytes)

Converting dates since 1792
from the Gregorian to the Jacobin calendar

Crane Brinton's discussion of the calendar The names of the months

Today's date:

Enter any date since 9/21/1792 to convert it to the French calendar:

picrep.gif        picrip.gif.png.jpg

The result:

This converter employs the Romme rule (see below) for handling leap years.

Why Conversion Programs Produce Varying Results

There are several internet programs which convert dates between the Gregorian and the French Revolutionary (Republican) calendars. These programs usually provide for the conversion not only of dates during the period when the French calendar was in use but also of dates subsequent to Napoleon's discontinuation of the calendar on Januray 1, 1806. Conversion of dates between 1792 and the end of 1805 is of utility to historians and genealogists. Conversion of subsequent dates is mostly of interest to hobbyists and francophiles (though the French Revolutionary Calendar was briefly revived by the Paris Commune in May 1871*).

Although the new calendar was not adopted until October 1793, the first day of the first year was retrospectively set for the day the Republic had been proclaimed, which happened to coincide with the autumnnal equinox in 1792: thus September 22, 1792 = 1 Vendémiaire I. A normal French calendar year of 365 days was to consist of twelve 30-day months followed by 5 "complementary" days, the "sans-culottides," at the end of the year, i.e. in late September.

The original scheme was to make every year start on the day of the autumnal equinox, as observed and calculated in Paris. To accomplish this, a leapday would be inserted as a sixth "complementary" day if it was needed to make the following year start on the autumnal equinox. This resulted in leap years in years III, VII and XI, with leap years projected for the years XV and XX. So leap years would be determined by an astronomical observation and, unlike the Gregorian calendar, would not occur at regular four year intervals. Note the five year interval between XV and XX.

However, the French declaration establishing the Revolutionary Calendar was itself somewhat ambiguous, even contradictory, in laying out a method for inserting leapdays in perpetuity, and a variety of proposed alternative methods arose. For example, the head of the commission which proposed the Calendar, Charles-Gilbert Romme**, subsequently suggested that leap year calculation be simplified by borrowing from the Gregorian calendar the 4-100-400 rule***, replacing astronomical observation with the simplicity of arithmetic and yielding a more regular cycle. Some of these alternatives are well discussed in Wikipedia.

Conversion of dates since January 1, 1806, is made problematic (and essentially speculative) by these differing methods. It is the differences among these methods which cause most of the apparently inconsistent results given by the programs on the internet.

There is another source of divergence, however: namely a defect in the implementation of the javascript interpreter on the Safari browser. It mishandles leap years in century years when using the Date(mm/dd/yyyy) function (although it gets 1900 and years divisible by 400 right). This defect exists even in the current version (6.0.4). The converter on this page avoids the problem with a small workaround, kindly provided by calendar maven Steve Morse.

* May 6th to May 23rd: 16 Floréal to 3 Prairial LXXIX.

** Romme's idea was at the time rejected, but it was eventually revived and serves as a popular method for handling French leap years following the year XX. Romme himself fell victim to the revolution, dying by his own hand on 29 Prairial III after being sentenced, along with five other Montagnards, to the guillotine.

*** "4-100-400 rule": A year is a leap year if it is divisible by 4, unless it is divisible by 100, unless it is divisible by 400.

Links to conversion programs with particular features

Stephen Morse's on-line program
Allows the user to select any of three defined methods for handling leap years. Allows conversions both to and from the French calendar.
José Luis Martin Mas's Dashboard Widget for the Mac
Converts (only) the current date to Revolutionary Calendar format, using the 4-128 rule for leap year calculations, i.e. a leap year is a year divisible by 4, unless divisible by 128.
José Luis Martin Mas's iPhone/iPad app: "Calendrier"
The user can choose between the Equinoctal rule and the Romme rule for leap year handling. ($1.99)

The names of the months

Vendémiaire Brumaire Frimaire Nivôse Pluviôse Ventôse Germinal Floréal Prairial Messidor Thermidor Fructidor
vintage mist frost snow rain wind seed blossom meadow harvest heat fruits

France.gif height=

Crane Brinton on the French Revolutionary calendar

The culmination...of revolutionary propaganda [was] its new calendar. Almanacs had been from the beginning of the Revolution a favorite and successful method of spreading the word. Collot d'Herbois himself had won, with his Almanach du Père Gérard, a prize offered by the Paris Jacobins for a work to spread the new ideas in simple language.

But for the Jacobins of 1794 it was not enough to print good republican moral counsels, after the manner of Franklin, at the appropriate dates and seasons. The whole calendar must be made over. The existing calendar perpetuated the frauds of the Christian church (Jesus himself was probably a good sans-culotte; all the nonsense stemmed from Paul), and was highly irrational and inconvenient.

The new calendar, based on a report of Fabre d'Églantine, was adopted by the Convention in October, 1793. By it the year began on September 22 of the old calendar, and was divided into twelve months of thirty days each, leaving five days (six in leap years) over at the end of the last month. These five or six days were to be known as the Sans-culottides, and were to be a series of national holidays. Each month was divided into three weeks, called décades, the last day of each décade being set aside as a day of rest corresponding to the old Sunday.

The months were grouped into four sets of three, by seasons, and given "natural" names, some of which are rather attractive--vendémiaire, brumaire, frimaire (autumn); nivôse, pluviôse, ventôse (winter); germinal, floréal, prairial (spring); messidor, thermidor, fructidor (summer). The days of the décade were named arithmetically--primidi, duodi, on to décadi. In place of the old saints' days, each day was dedicated to a suitable fruit, vegetable, animal, agricultural implement.

The Sans-culottides were dedicated, the first to Genius, the second to Labor, the third to Noble Actions, the fourth to Awards, and the fifth to Opinion. This last was to be a sort of intellectual saturnalia, an opportunity for all citizens to say and write what they liked about any public man, without fear of the law of libel. The sixth Sans-culottide of leap years was dedicated to the Revolution, and was to be an especially solemn and grand affair. The republican era was to date from the declaration of the republic in September, 1792. When the calendar came into use, the year I had already elapsed.

In spite of its symmetry and its poetic months of budding and of mist, the new calendar was not a success, and Napoleon abandoned it....Workingmen preferred one day's rest in seven to one in ten; its terminology, appropriate to the climate of France, was singularly inappropriate to that of the Southern Hemisphere; it embodied a new cult, and that cult, though it profoundly influenced Christians then and since, failed completely to supplant Christian terminology. The calendar and its fate form in many ways a neat summary of Jacobin history.

--from A Decade of Revolution, 1789-1799 (1934)

link to Traction Avants in Northern California

Peter Windhorst, M.D.
Menlo Park, California

visits since 1998: