Any function on a watch beyond merely telling time is called a “complication”, and it’s a fitting term. Even adding a date function involves the precise addition of gears and linkages to make that number flip over every 24 hours. Aside from the mechanical wizardry of watch complications, there is also a rich history intertwined with their development. Over the next several months, our “It’s Complicated” series will examine seven of the best-known watch complications — the chronograph, the moonphase, the annual calendar, the minute repeater, the GMT, the world timer and the perpetual calendar — explaining how they work, their history and a handful of modern timepieces that best exemplify them. – Jason Heaton

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f you glance at your watch on July 1st and find that its calendar still shows the 31st, you can thank the Romans. In the earliest days of their fallen empire, their calendar followed a fairly simple lunar cycle with an arbitrarily set 10 months. When Roman astronomers proposed a more accurate solar calendar with 12 months, the two new ones were named to honor the Caesar dynasty, Augustus and Julius, and immediately rendered September (meaning “seventh”) through December (“tenth”) misnamed. Naturally, these new Caesarian months had to have the most days (31), and so did October (Caesar Octavius). So while the calendar approximated the sun’s travel across the sky, these imperially decreed longer months would forever become the bane of future watchmakers since a simple watch with a date function is not smart enough to distinguish months with 30 days from those with 31. And that’s not even to mention the problem of February.

The concept of the leap year was also the result of a decree, though religious instead of political. Pope Gregory wanted to ensure that Easter fell as close as possible to the spring equinox every year so he declared that every fourth February will get an extra day to keep the calendar synchronized to the seasons. This so-called “Gregorian” calendar compensates for the fact that our 365-day year is six hours shorter than a true solar year and so “resets” it every four years. As if the Romans’ narcissistic calendar wasn’t enough to have to deal with, the Gregorian one added even more complexity for future horologists.

Horological Orgy

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Named for its original New York meeting place, Red Bar is a group of watch enthusiasts who meet regularly to share their passion for all things horological. While that original dive bar is no longer, the name lives on as the group meets weekly in an undisclosed location in Manhattan to talk, drink, post gratuitous photos on Instagram and watches have been known to change hands. Led by watch impresario and erstwhile GP contributor, Adam Craniotes, Red Bar is far from a stuffy bunch and decidedly irreverent; arrive at one of their meetups, check your ego at the door and be prepared to pass your watch around. Red Bar has become so popular that watch brands and retailers have taken notice, some attending events to show off their wares and others hosting events for group members. Satellite Red Bar groups have sprung up in cities across the country and even across borders, in cities like Los Angeles, Minneapolis and Toronto. The meetups aren’t publicized for obvious reasons so if you want to attend, you’ve got to know someone.

The most elemental form of calendar complication is found in a watch with a date function, which dutifully marches off 31 24-hour cycles before starting all over again thanks to its system of gears that are toothed to coincide with most months. But with the addition of a set of gears and discs driven off the going train of the movement, the possibilities grow, and in the 1800s, complete, annual and perpetual calendar watches became popular not only for their usefulness but also for their complexity. These watches add the day of the week and the month and, in their most complicated form, the leap year cycle, allowing the watch to correct itself for not only short months but also for those years that Pope Gregory wanted to be a day longer.

Calendar watches can be divided up into three categories of increasing complexity. A complete calendar, sometimes called a “full calendar”, is a watch that displays the day of the week, the date, the month of the year and, of course, the time. Often these watches also show the phase of the moon. Consider the complete calendar an entry point or a gateway watch to high complications. They are classically elegant and useful but more affordable than their more elevated cousins since they have less going on under the hood. With a complete calendar watch, you must still advance the date on those months that have less than 31 days, usually with a small button on the side of the case.

As if the Romans’ narcissistic calendar wasn’t enough to have to deal with, the Gregorian one added even more complexity for future horologists.

A step up from the complete calendar brings much greater functionality and an accordingly higher price. An annual calendar watch has the same display as a complete calendar but is “smarter” in that it compensates for the short months of the year, besides pesky February, by advancing accordingly. An annual calendar has got you covered for 11 of the 12 months of the year and three out of every four years; as long as you keep it wound, you won’t have to touch it to correct the date March through January.

At the top of the pyramid, not only for calendar watches, but for any kind of watch, is the perpetual calendar, or quantième complet. In the watchmaking pantheon, the perpetual calendar is considered a “grand complication”,revered by collectors and coveted by enthusiasts who wish they were collectors.

The perpetual calendar is, as its name suggests, self-sufficient in its date calculations, even in leap years, thanks to a “mechanical memory” of 1,461 days. Set it and forget it. Just keep it wound or there’s a considerable amount of setup you’ll have to do to get it back in synch. Perpetual calendars maintain the classic layout of the other calendar watches, with day, date, month, time and often moonphase displays, but usually add an indication of the current position in the four-year leap year cycle.

There is one calendar anomaly that will foil even the mighty perpetual calendar, and again the Romans are to blame. On March 1st of the year 2100, it will not be a leap year, unlike all the others before it. This is due to a quirk of Pope Gregory’s ancient calendar, which says that three century years out of four are not to be leap years. If you’re lucky enough to own a perpetual calendar, leave your grandson a note.

Here are some calendar watch choices to keep you on schedule and channel your inner Roman. Not all pair well with a toga.

IWC Portugieser Annual Calendar

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IWC debuted the new Portugieser Annual Calendar at the 2015 SIHH gala with an all-new movement and contemporary good looks. The movement, developed in house in Schaffhausen, has a full seven-day power reserve and shows the month, date and day in the “American” style (month first) in deference to IWC’s American founder, F.A. Jones.

Girard-Perregaux 1966 Full Calendar

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The 1966 family of G-P watches is about as classic as you can get. With its 40mm pink gold case, crisp white dial, minimalist stick markers and an in-house movement from one of the best names in the business, you won’t mind that you have to correct this watch’s date every few months.

Baume & Mercier Clifton 10057

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Baume & Mercier is the king of accessible high watchmaking. The Clifton has the elegant lines and functionality of the heavy-hitters in this category but comes in at about half the price. This is thanks to an outsourced movement that adds some heft to the case, but it’s still a watch your son won’t turn up his nose at when you pass it down.

Jaeger-LeCoultre Master Calendar Meteorite Dial

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JLC is often called “the watchmaker’s watch company” due to its huge catalog of in-house calibres, many of which have been used by other, more prestigious brands. Nobody does complications like Jaeger, and its Master Calendar is no exception, moving the date to the outside of the dial and keeping overall height to less than 11mm. The meteorite dial version uses material from a sliver of a meteorite that crashed into a Swedish glacier centuries ago.

A. Lange & Söhne Saxonia Annual Calendar

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The German masters at Lange are known for many things but its most recognizable trademark is the outsized date, modeled after the clock in the Dresden opera house. Instead of showing the day in an aperture on the dial, it is pointed to on a subdial, as is the month, giving the watch a clean, Teutonic aesthetic.

Georg Jensen Koppel Annual Calendar

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Georg Jensen is better known for silversmithing, but the brand has recently made a concerted effort to produce timepieces befitting its Danish design heritage. The Koppel Annual Calendar veers from the classic norms of this watch category with a clean enamel dial, a small seconds subdial and a minimalist approach to displaying the calendar, using an offset arc-shaped aperture for the month and the date front and center. In addition to being possibly the most beautiful selection, it is also the most affordable at less than $6,000.

Patek Philippe 5140

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No listing of calendar watches or grand complications is complete without a Patek, and the 5140 is one of the brand’s most iconic timepieces. The sheer weight of Patek’s legacy alone is enough to make this watch significant, and its classic subdial arrangement of all relevant calendar details is reminiscent of every Patek perpetual calendar dating back to the turn of the last century. Then you turn it over and gaze on the exquisitely decorated movement, and you can see why Patek sits at the top of the heap.

Breguet Classique 5327

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Breguet’s founder, Abraham-Louis Breguet, made watches for Napoleon and Marie Antoinette and was behind most of watchmaking’s milestones during his lifetime. But his company’s legacy continued long after his death, and in fact they built the first perpetual calendar wristwatch in 1927. The Classique 5327 carries forward many of the brand’s longstanding aesthetic cues, with blued hands in a style named for Breguet himself, a guilloche-patterned dial and distinctive subdial layout for month, day, date, moonphase and leap year. While this is a classic piece, it is also thoroughly modern, with a silicon hairspring that is immune to the detrimental effects of magnetism. Old Abe would have been proud.

IWC Big Pilot Perpetual Calendar Top Gun

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Most calendar watches are dainty affairs, well suited for wear under a starched monogrammed shirt cuff — but not this one. Try a flight suit. The undisputed brute of the category is the ceramic-cased Big Pilot Top Gun from IWC, which is one grand complication you can wear for all the 1,461 days of the leap year cycle with little fear of damaging it. It features a full digital year display (an IWC invention), and all calendar functions can be set via the crown, unlike other watches of its kind that use various push-piece correctors on the case flanks. It also shows the moonphases in both the northern and southern hemispheres, in case your carrier sorties take you across the Equator.