A perpetual calendar is a calendar system that can accurately display the date for many years, often centuries, without needing adjustment. It accounts for the varying lengths of months and leap years, making it useful for tracking dates over long periods. These calendars can be found in various forms, including mechanical devices and software applications. They are often used in fields like astronomy and history, where precise date calculations are essential. A well-known example of a perpetual calendar is the Gregorian calendar, which is the most widely used civil calendar today.