An Annual Calendar complication is designed to display the day, date (or sometimes just date, which is the key function) and month with minimal adjustment. The complication automatically adjusts the date displayed on the timepiece based on 30 and 31-day months, so it only requires one annual adjustment: setting the date at the end of February.
The annual calendar is a fairly recent development in watchmaking; it didn’t appear until 1996, when Patek Philippe introduced the Ref. 5025. The annual calendars here—all introduced this year—run the gamut from the mid-level (the Rolex Sky-Dweller in Oystersteel Annual Calendar, $14,400) to the elite (the $151,000 Richard Mille RM60-01).