French Republican Converter

The National Convention replaced the calendar in 1793, backdated to 22 September 1792 — the day the Republic was proclaimed and, by convenient coincidence, the autumn equinox. The year gained twelve months of thirty days, with names Fabre d'Églantine drew from weather and harvest: Vendémiaire from the grape harvest, Thermidor from heat, Brumaire from mist. The five days left over went to the end of the year, outside the months.

The change that weighed heaviest was not the month names but the week. Each month became three décades of ten days, with the days numbered Primidi to Décadi — and rest went from one day in seven to one in ten. For a working person that meant losing two days off a month, which made the calendar one of the most unpopular revolutionary measures among the very people it was meant to serve.

The conversion holds a subtlety almost every converter gets wrong. The original decree set the start of the year by the true autumn equinox in Paris, not by an arithmetic rule; the fixed rule Gilbert Romme proposed was never enacted. The 366-day years actually observed were III, VII and XI, which corresponds to years congruent to 3 modulo 4 — and it is that rule, not an average of 365.25 days, that reproduces all fourteen documented dates exactly.