
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 READ MORE