The French Republican Calendar was created after the revolution to remove all religious references from the calendar, but how were years numbered on it? The Gregorian Calendar (which was used before the Revolution and after the Reign of Terror) numbers years starting from the birth of Christ, so it seems implausible to me that the radicals in the government would keep that dating system in use. How were years numbered on this calendar?
The French Republican Calendar started counting years from 22 September 1792, the day the French First Republic was proclaimed. Year I was the first year of the Republic, and so on.
You can read more about the calendar's design on Wikipedia.