The perpetual calendar is one of the best known complications in watchmaking and one of the most useful: it’s a miniature computer that automatically corrects the date at the end of each month, since some months (as the children’s rhyme reminds us) are 30 days long, and some 31. The odd man out is February: 28 days long except in a leap year, when an extra day is added to correct for the fact that a standard year is actually about 365 and a quarter days long. (The whole story is much more complicated than that but we can leave that to the astronomers.)
It’s a fantastically useful but also fantastically complicated mechanism and most perpetual calendars are rather more delicate than not –and, they are usually quite difficult to set, requiring the correct operation of four different correctors set into the case of the watch. Not only that, they’re notoriously intolerant of rough handling. To fill the gap, the annual calendar was developed; the annual calendar doesn’t automatically correct for February but it does for all the other months, and in exchange for the negligible trouble of setting the watch from the 28th of February to the 1st of March by hand once per year, it does exactly what the perpetual calendar does.