The annual calendar has the ability to take into account the alternation between the months with 30 and 31 days. It must be manually set only in February, as it is the shorter month of the year. Annual calendars are then more refined than simple calendars, but not as complex as the perpetual ones. Some professionals say they became so successful these last years because perpetual calendars are particularly demanding and delicate to set. Annual calendar represent then a much more acceptable alternative that has still some interesting features technically speaking. In addition, it is simpler and cheaper to build and assemble. All these points explain then why annual calendar are also priced slightly above the simple calendars but not as expensive as the perpetual one. In fact, they filled some kind of gap between the basic ones and these. Historically speaking, the first annual calendars on wristwatches appear during the 80’s. But they become really known in the public when the powerful Patek Philippe introduces its own one in 1996. The brand had entirely rebuilt it and since then the annual calendar became a very appreciated and typical complication from the manufacture. During about ten years, Patek Philippe has some kind of exclusivity on this function. But since 2006, the annual calendar is adopted by dozen of brands. Its usability and superior reliability considering its relative complexity must have had a strong influence

Speaking more in detail about technique now, let’s mention that an annual calendar can count between a few components up to more than 150. The Patek Philippe one reaches 154 parts for instance. As the rule in watchmaking is to simplify at the maximum mechanisms to gain time, money and reliability, some kind of competition emerged to create the simplest annual calendar. And one major inventor is Ludwig Oechslin, former Director of the International watchmaking museum in La Chaux-de-Fonds. Oechslin must be seen not as a pure brilliant thinker but also as a practical man deeply concerned by simplifying the mechanisms he worked on at the maximum. He invented then a mechanism with 4 dedicated wheels. The giant Rolex also developed a system called Saros, which describes a cycle of perfect eclipse occurring every 18 years approximately. The Saros works differently and counts as well 4 moving parts involved in its run. 

So if you have to remember one thing about the annual calendar, it should be that it is a practical and useful one that needs to take car of only one time a year.