In order to understand why the secular perpetual calendar is so impressive, you need to understand how the Gregorian calendar works across centuries of time. Unsurprisingly, it all comes down to leap years – the phenomenon whereby every four years an additional day is added to the end of February. However, there’s also a nuance whereby in centenary years there is only a leap year if the year is also divisible by 400. So, the year 2000 was a leap year but the years 2100, 2200 and 2300 won’t be, and then the year 2400 will be. A standard perpetual calendar will assume that all of these years are leap years, which is why you’ll often see brands saying the next time their perpetuals need updating is the year 2100.
A secular perpetual calendar can account for these centenary years, skipping three leap years across a 400 year period. That means they will stay accurate to the Gregorian calendar indefinitely. In fact, the only limitation and the reason brands can’t advertise that their watches will be accurate forever, is that humanity itself hasn’t decided if the year 4000 will be a leap year or not. These watches are even rarer than perpetual calendars and we don’t exactly have perpetual calendars coming out of our ears.
