Why is anniversary clock called a 400 day clock?

An anniversary clock is called a 400-day clock because it is designed to run accurately for approximately 400 days on a single winding. This unique timekeeping mechanism relies on a torsion pendulum that oscillates, allowing the clock to maintain precision while minimizing the need for frequent winding. The term "anniversary" reflects the clock's ability to run for nearly a year and a bit beyond, making it a popular gift for significant milestones.