It’s 1 Vendémiaire of An I in the French revolutionary calendar, the first day of the first month of the first year of the First Republic. But no one would know about it for another year, when the calendar was imposed retroactively.
Conversion to the new metric system was already under way (see here), and the Gregorian calendar (see here) was considered a vestige of the ancien régime. The new calendar became law in autumn 1793 but officially began September 22, 1792: birth date of the republic and, conveniently, the autumnal equinox.
The year comprised twelve egalitarian months of thirty days each. The week was abolished. Months were divided into three décades of ten days each, numbered from one to ten. The tenth day was a day of rest, leaving nonagricultural workers with only three days off a month. Some folks resented that. Five extra days (outside any month) at the end of each year were celebrated as holidays. Leap year (see here) added a sixth.
Revolutionaries even attempted a metric day: ten hours of one hundred minutes each, each minute being one hundred seconds. The metric second was 14 percent shorter than the one we know. It was even less popular than the calendrier révolutionnaire: clocks and watches are considerably more expensive than paper calendars and ledgers.
The republican calendar was a major bother to diplomats and international merchants. They needed cumbersome conversion charts that listed each day next to its counterpart in the other calendar. Emperor Napoléon, who’d already abolished the republic itself, abolished the republican calendar in 1805. The Gregorian calendar resumed throughout Napoléon’s empire on January 1, 1806, aka 11 Nivôse An XIV.
Historians labored with conversion tables for another two centuries, until the World Wide Web (see here) came along with its handy conversion engines.—RA