The Coligny Calendar is a Gallo-Roman luni-solar calendar with a 30-year metonic cycle likely compiled by Druidic leaders in the first century BCE, based partly on earlier Babylonian calendars. Discovered in 1897 near Coligny, France, the bronze plaque fragments show evidence of centuries of adjustments to align lunar and solar years, revealing insights into Gallo-Roman timekeeping science. The calendar divides the year into twelve named months plus intercalary months, with each month split into two fortnights tracked by movable pegs.