![]() Several complex perpetual calendar watches include leap year indicators, day-and-night indicators, moon phase displays, and so on, as seen in this IWC Portugieser Perpetual Calendar Given these rules, the next exceptional non-leap year will be 2100. The math behind it defined the current Gregorian calendar in 1582, and we’ve been following these leap year rules ever since. To tackle that, the centurial years that are not divisible by 400 are exempted from being leap years. There is still a difference though, and if we were to only have leap years, and no other correction in the calendar (which is how it was before the year 1582), there would be an 11-minute shift in the calendar every year. So a leap year is, in theory, any year that’s perfectly divisible by four. It takes 365.2421897 days, but we round that off to 365.25, allowing four quarters of a day to come together and give us a 29th day in February, once every four years, making it a leap year. That’s not too bad though, is it? The reason for this is the unrounded number of days that the planet takes to go around the sun. So instead of forever, we have to settle for seeing the correct date on our perpetual calendar watches only until Macomes along. However, the revolution of planet Earth around the sun and the Gregorian calendar make things a little too complicated for this complication to handle. ‘Perpetual’ implies forever, and that’s how long a perpetual calendar watch is supposed to tell the correct date without adjustment. ![]()
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