During the French Revolution, the government replaced the traditional calendar with a new system based on logic and nature.

The French Republican Calendar was officially adopted by the National Convention on October 24, 1793, though its epoch was backdated to the start of the Republic on September 22, 1792. It was designed by a commission including mathematician Gilbert Romme and poet Fabre d'Églantine. The goal was to de-Christianize France and implement a decimal system for time, reflecting the same Enlightenment logic that created the metric system.Each year consisted of 12 months, and each month was exactly 30 days long. To align with the solar year, five or six 'complementary days' were added at the end of the year as national holidays. The traditional seven-day week was replaced by the 'décade,' a ten-day period where only the tenth day was a day of rest. This change was particularly unpopular because it forced laborers to work nine days straight instead of six.The months were given poetic names based on nature, such as 'Brumaire' for fog and 'Thermidor' for heat. However, the system created chaos for international commerce and diplomacy since the rest of the world still used the Gregorian calendar. After just over 12 years of use, Napoleon Bonaparte signed a decree on September 9, 1805, which officially reinstated the Gregorian calendar on January 1, 1806.