Few complications boast the same romanticism and functionality as the perpetual calendar does. Watches with a calendar function require constant adjustment to ensure the correct date is shown, and watches will usually reset after completing 31 24-hour cycles — a full month. What that doesn’t take into account, however, is that the concept of time as we’ve created it isn’t as simple as that; we can’t just ignore the irregular number of days per month, much less the leap years.

To counter the inconsistencies of time, watchmakers set about to develop more advanced complications, which track the calendars with higher accuracy and without the need for monthly adjustments. There’s the annual calendar, which adjusts the date displayed based on 30- and 31-day months automatically, but it requires the manual setting of the date once a year, at the end of February.

So the horologists got back to work and the result was the perpetual calendar. Like its name suggests, the complication has a mechanical memory programmed for a very long time — it will indicate the correct date for a century according to the Gregorian calendar, taking into account the leap years without requiring manual correction. The only exception is when the secular year is divisible by 400. This means that three century years out of four will not be a leap year, so the proud owner(s) of a perpetual calendar watch would still be inclined to correct the date three times over the five centuries on 1 March.

With one of the most classical complications ever created, perpetual calendar watches have enamoured even the most discerning of powerhouses and aficionados. Here are 5 that you can start looking into owning now.