Most watches that contain a calendar function require constant adjustment to ensure the correct date is displayed. These watches will reset after completing 31 24-hour cycles, representing a full month. Since our calendar contains leap years and an irregular number of days per month, watchmakers developed two advanced complications to track our calendars more accurately so that the date doesn’t have to be adjusted every month. As long as the timepiece is wound and running, the calendar complication will be functional and accurate.
An annual calendar complication is a complete calendar displaying day, date, and month with minimal adjustment. The complication automatically adjusts the date displayed on the timepiece based on 30- and 31-day months to ensure that the correct date is always displayed. This complication requires setting the date once per year, at the end of February.
The perpetual calendar complication also displays the day, date, and month, but it requires even less adjustment than an annual calendar. This complication not only knows the number of days in each month, but it also knows the year so that it will automatically adjust for the leap year. The next time a perpetual calendar will have to be adjusted is in the year 2100.