Of all the complications in the world of watchmaking, few have the glory and prestige of a true perpetual calendar. Regarded as one of the most challenging complications to design, assemble, and service, a good perpetual calendar's functionality will outlive that of its original owner quite comfortably if kept in good working order.
A perpetual calendar sounds a lot more complicated than it is (but nowhere near as complicated as it is to make and make well). Simply put, it is a calendar that shows the date, the day, the month, the year, and the leap year at once, while able to automatically compensate itself for months of varying lengths.
The refinement of these mechanisms can be so preposterously accurate that some of the finest examples will remain accurate up until 2100 (a "Non-secular year").
Secular years occur once times every 400 years, in "century" years that are divisible by 400. So, for example, the years 1700, 1800, 1900, 2100, 2200, and 2300 are not secular years, while the years 2000 and 2400 are.
In secular years the leap year is observed as it normally would be every 4 years. In non-secular "century years", the leap year is omitted and February remains just 28 days long. Unfortunately for everyone alive on Planet Earth at this moment in time, this is unlikely to be something any of us will experience (unless cybernetics really take-off in the next century).
This is the one serious anomaly in the Gregorian calendar for which Perpetual calendars can not compensate for. Horology has seen secular perpetual calendars created but they are exceptionally rare and not something wristwatch manufacturers seem wholly focused on producing (due to the space needed for the extra components that would only be called into action once every four centuries).
If the watch is kept wound (and remains running) a perpetual calendar needs only to be adjusted by one day every time we reach a non-secular century year. That's quite a responsibility for the owner, but it isn't a disaster if the watch is allowed to run down its power reserve.
Although setting a perpetual calendar (forwards) is doable, it is one of the most complicated complications to set-up. This made the release of the MB&F Legacy Machine Perpetual, designed by Stephen McDonnell, a seminal moment in the history of perpetual calendars as, for the first time, a truly straightforward and intuitive setting mechanism had been realized in a wristwatch.
And that history is a long one. It stretches back to 1762 when famous British horologist Thomas Mudge conceived the complication. It would remain clock-bound until the early twentieth century when Patek Philippe became the first watchmaker to successfully install a perpetual calendar in a wristwatch in 1925.
Perpetual calendars are still relatively rare, and it is more common to see brands in the Haute Horlogerie sphere producing more visually arresting, but technically simpler complications like a tourbillon, an advanced moon phase complication, or a creatively arranged chronograph or GMT. When a brand releases an in-house perpetual calendar it makes a very strong statement about the brand's manufacturing ability, and while the high price tag attached to even the most affordable perpetual calendars means they will never be big volume sellers, they are very useful in establishing a brand's position in the high-end of the market.
With the longest history of any brand when it comes to perpetual calendars in wristwatches, Patek Philippe has several very handsome models on offer. Hot on the heels of Patek Philippe's 1925 release was Breguet, which followed suit in 1929 debuting the brand's very first perpetual. Still, to this day, Breguet produces some very nice models in this style.
But for one of the most affordable perpetual calendars ever produced, check out Frederique Constant. By grafting an in-house module on top of an existing in-house movement, the brand was able to produce a very complicated watch for around $9,000 - a true statement of intent for the future, and a very exciting one for all other medium-sized brands looking to make a serious horological impact.