From 1793-1805 France used a different calendar from the rest of the world, until Napoleon abolished it and returned France to the Gregorian Calendar for the start of 1806. French Revolutionary Calendar on Wikipedia.
Mathematical Calculator
The calculator below is an expanded version of a script which I developed for The Voice of the Turtle. It will convert Gregorian dates after September 22nd 1792, the official start of the calendar, into the revolutionary calendar. It is based on the mathematical calculation of leap years proposed in 1795 but never used.
Historical Calculator
This second calculator will convert revolutionary dates for years I to XIV (the years the calendar was actually in use) into the Gregorian calendar. It used the astronomical system for determining leap years that was actually used.
As a consequence of the two different systems, there were leap years in years III, VII and XI which is reflected in this calculator, whilst in the calculator above the leap years fall in years IV, VIII, XII, XVI, etc. Hence the two calculators will sometimes give results a day apart.