It ticks once a year, the century hand advances once every 100 years, the cuckoo comes out on the millennium, and it will keep time for the next 10,000 years. https://www.messynessychic.com/2020/09/18/why-is-someone-building-a-10000-yr-clock-inside-a-mountain-the-10000-yr-anti-doomsday-clock/