Revolution (Hong Kong) - - TIME COMPLICATION - By jaCK FORSTER

Time marches on, but it does so in a reg­u­larly ir­reg­u­lar way; and so, we have the var­i­ous forms of cal­en­dars by which mankind has for mil­len­nia tried to cope with the fact that a year is not evenly di­vis­i­ble by a whole num­ber of days. There are, as it turns out, a lot of dif­fer­ent ways to de­fine a year, but the two we’re in­ter­ested in as watch lovers are the trop­i­cal and the side­real year. Ba­si­cally, both of th­ese have to do with how long it takes the sun to re­turn to a cer­tain start­ing point in the sky as seen from Earth, but the trop­i­cal year’s usu­ally the one used as a start­ing point for cal­en­dars be­cause it has to do with how long it takes the sun to cross the point in the sky mark­ing the ver­nal (or spring) equinox. (For the side­real year, you use the back­ground of stars as a ref­er­ence in­stead — it’s sim­i­lar to the dif­fer­ence be­tween a so­lar and side­real day.)

The ba­sic idea is that since you can’t fit a whole num­ber of days into a year, any cal­en­dar you can pos­si­bly come up with is go­ing to start to drift out of syn­chrony with the sea­sons, which is why our an­ces­tors came up with the bril­liant idea of hav­ing years of dif­fer­ent lengths. The ba­sis of any self-cor­rect­ing cal­en­dar — like your fa­vorite and mine, the Gre­go­rian cal­en­dar — is that you start out with a cal­en­dar that is ac­tu­ally a lit­tle shy of a true year in length. If a year is ac­tu­ally around 365.25 days long, then you’ll be a full day short at the end of four years. If, how­ever, you are clever, you just make sure that ev­ery four years, you add an ex­tra day to the cal­en­dar. This keeps ev­ery­one happy be­cause it means that the date doesn’t grad­u­ally drift out of synch with the sea­sons, and that April show­ers will al­ways bring May flow­ers, in­stead of April snow bring­ing… May snow, de­pend­ing on your lat­i­tude.

(The ac­tual ex­act length of a trop­i­cal year? Bad news: there isn’t one; it varies due to count­less tiny ce­les­tial wob­bles like the rate of pre­ces­sion of the Earth’s axis. The mean trop­i­cal year, on 1 Jan­uary 2000, was 365 days, five hours, 48 min­utes, and 45.19 seconds. As you can see, 365-and-a-quar­ter days is, as they say, close enough for gov­ern­ment work.)

Now, con­sider the watch (or clock). A ba­sic cal­en­dar in­di­ca­tion is pretty sim­ple,

re­ally. You’ve al­ready got the hour hand go­ing around twice a day. All you have to do is put a re­duc­tion gear on the hour-hand wheel that turns that into one ro­ta­tion a day, and then you use that gear to ad­vance the date one day ev­ery 24 hours (you can use a hand or a disc, or what­ever you like — we’re con­cerned here with me­chan­ics, not cos­met­ics).

As long as the month is 31 days long, you’re good to go; if it’s shorter, you have to ad­vance the date man­u­ally on the 30th (if it’s Septem­ber, April, June, or Novem­ber) or on 28 Fe­bru­ary (in a non-leap year) or 29 Fe­bru­ary (in a leap year). (At this point, some peo­ple might ask why you would need an ex­tremely com­plex and del­i­cate mech­a­nism to save you the un­speak­able agony of hav­ing to spend 30 seconds turn­ing the crown a cou­ple of rev­o­lu­tions five times a year, but let us not con­cern our­selves with such benighted per­sons.)

Gen­er­ally, the way a per­pet­ual-cal­en­dar watch han­dles this is through the use of some­thing called a “pro­gram wheel”, which is a kind of gear (rather large com­pared to other gears in a watch) that has 48 el­e­va­tions and de­pres­sions on it. The tip of a lever rests on one of th­ese 48 points, and, de­pend­ing on the depth, will make the cal­en­dar ad­vance the date ei­ther one, two, three or four days at the end of the month. (The big­gest jump cor­re­sponds to the deep­est notch, and if you look at a clas­sic per­pet­ual-cal­en­dar mech­a­nism — a lit­tle dif­fi­cult to do un­less your watch has a trans­par­ent dial, as the cal­en­dar works are usu­ally un­der the dial, and thus not vis­i­ble through the dis­play back — you’ll see a to­tal of four deep notches for the four months of Fe­bru­ary, with one slightly less deep than the oth­ers; that’s for Fe­bru­ary in a leap year, when we add 29 Fe­bru­ary.)

The in­ven­tion of the per­pet­ual cal­en­dar is gen­er­ally cred­ited to Breguet, although in­ter­est­ingly enough, one of Breguet’s ear­li­est per­pet­ual cal­en­dars didn’t use the abovedescribed sys­tem. Dr. George Daniels gives, in The Art of Breguet, a date of 1795 for the in­ven­tion of the per­pet­ual cal­en­dar, and the most fa­mous ex­am­ple of a Breguet watch with a self­cor­rect­ing cal­en­dar is Breguet no. 160, bet­ter known as the “Marie-An­toinette”. The his­tory of the watch is as com­pli­cated as the watch it­self, but its per­pet­ual cal­en­dar is of in­ter­est here, thanks to its use of a very clever — if very com­pli­cated — de­sign that’s driven by the kid­ney-shaped cam for the equa­tion of time (EOT). In this de­sign, the EOT cam, which turns once a year, has four pins on it that strike a lever con­trol­ling the num­ber of days ad­vanced at the end of the month; when the lever’s po­si­tion is changed by one of the pins, the date changes from the 30th to the first (four pins for the four months of 30 days’ length). For the month of Fe­bru­ary, there is a four-pointed star wheel with four pins at­tached to each tooth, also mounted on the EOT cam. Th­ese pins con­trol the same lever as the ones at­tached di­rectly to the EOT cam. Three of the pins are set so that the date ad­vances from 28 Fe­bru­ary to 1 March, and one is set so that, once ev­ery four years, the date jumps from the 29th to the first. The star wheel turns one full revo­lu­tion ev­ery four years.

It’s an ex­tremely clever mech­a­nism, and ob­vi­ously lends it­self to sim­pli­fi­ca­tion. The 48-step pro­gram wheel was most no­tably used in the early 19th cen­tury by the French watch and clock­maker, Achille Bro­cot, and is ob­vi­ously con­cep­tu­ally con­nected to the

mech­a­nism Breguet used for the Marie-An­toinette (the lat­ter was fur­ther com­pli­cated by Breguet’s use of a ret­ro­grade date hand). How­ever, Breguet’s idea of us­ing a four-pointed star wheel mounted on the pro­gram disc was res­ur­rected to­ward the end of the 19th cen­tury, to re­duce the num­ber of steps on — and thus the size of — the pro­gram wheel.

Again, this is pretty sim­ple if you think about it — the only rea­son you need 48 steps to be­gin with is be­cause you have, once ev­ery four years, an ex­tra day on the month of Fe­bru­ary. If you use the star-wheel sys­tem, you can get away with a pro­gram wheel with only 11 steps, plus the point on the pro­gram wheel where you put the star wheel.

This sys­tem ap­pears in a patent granted to Patek Philippe in 1889. The ma­jor dif­fer­ence is that, rather than a star wheel, the Patek de­sign used a four-faced block mounted on a Mal­tese cross, which is placed on the now 11-step (plus the block) pro­gram wheel. Once ev­ery four years, the block is turned so the lever con­trol­ling the length of the month is rest­ing on a face slightly higher than the other three — and, hey presto, the date ad­vances to the first on Fe­bru­ary the 29th, rather than the 28th. Us­ing this sys­tem, it’s pos­si­ble to re­duce the di­am­e­ter of the pro­gram wheel con­sid­er­ably.

So mat­ters stood for some time. The per­pet­ual-cal­en­dar mech­a­nism, at least in its clas­sic ver­sions (whether the sort used by Breguet for the no. 160, the Bro­cot de­sign, or the type shown in the Patek patent of 1889), was not a common com­pli­ca­tion. It wasn’t so much that there were a lot of parts per se, but rather that the com­pli­ca­tion (which was, and is, of­ten com­bined with a moon­phase) has a num­ber of blade springs, jumpers (springs with a tooth on the end that holds a wheel in place un­less it’s specif­i­cally meant to move) and levers that are rel­a­tively dif­fi­cult to make. The clas­sic ver­sions are also del­i­cate, eas­ily dam­aged if mis­han­dled, and trou­ble­some to set cor­rectly, as each of the in­di­ca­tions has to be ad­justed through push­ers in the case (man­u­fac­tur­ers of­ten give you a nice lit­tle sty­lus for this pur­pose, which you will, of course, pro­ceed to lose im­me­di­ately, and there is some­thing jar­ring about ad­just­ing your me­chan­i­cal paean to the high­est pin­na­cle of horo­log­i­cal tra­di­tion with a tooth­pick). For many years, the per­pet­ual cal­en­dar was rare enough that this didn’t mat­ter very much, and it of­ten trav­eled along with other com­pli­ca­tions in pocket watches made for ul­tra-wealthy and ul­tra-se­ri­ous col­lec­tors — King Farouk, Henry Graves, James Ward Packard — who were not go­ing to be de­terred by lit­tle things like case push­ers (and who most prob­a­bly had some­one on staff full-time just to at­tend to their watches). But even­tu­ally, the wrist­watch be­gan to sup­plant the pocket watch.

The very first per­pet­ual cal­en­dar to find its way onto a wrist, at least as far as any­one can tell for cer­tain, was made by Patek Philippe, and it started out as a move­ment com­pleted in 1898 and orig­i­nally in­tended for a ladies’ pen­dant watch. In 1925, how­ever, Patek Philippe placed the move­ment in a wrist­watch case, as ref. 97975, and two years later sold it to a well-known Patek client named Thomas Emery (Patek later bought back the watch from his es­tate and it now re­sides in the Patek Philippe Mu­seum). The watch also fea­tured an in­stan­ta­neous jumping mech­a­nism for the per­pet­ual cal­en­dar (the main sub­ject of Patek’s patent from 1889).

In­stan­ta­neous jumping of per­pet­ual-cal­en­dar in­di­ca­tions makes an al­ready chal­leng­ing com­pli­ca­tion even more so, as the en­ergy nec­es­sary to jump all the in­di­ca­tions at mid­night has to be drawn from the main­spring. The tra­di­tional way to do this is to put a cam on the drive wheel for the per­pet­ual cal­en­dar, with a lever press­ing against it un­der the force of a fairly strong straight spring. The cam works in the same way as the re­turn-to-zero cam on a chrono­graph; at mid­night, the tip of the lever rides over the high point of the cam and in­stantly (well, prac­ti­cally in­stantly) ro­tates the cam so that the lever now rests on the low­est point. This causes a foot mounted on the cam to flick the date-change lever, and the in­di­ca­tions all switch over. The sys­tem must have been dif­fi­cult to minia­tur­ize at first — for one thing, it takes a lot

of en­ergy, rel­a­tively speak­ing, and shrink­ing ev­ery­thing down to wrist­watch size was en­ter­ing terra incog­nita. The at­trac­tion of such a mech­a­nism must have been even greater, though, be­cause the first wrist­watch move­ment we know of that was re­ally de­signed from the ground up as a per­pet­ual cal­en­dar, was made by Breguet and com­pleted in 1929: the Breguet no. 2516, sold in 1934 to a Mon­sieur Jean Doll­fus. (Jean and his brother Louis were Breguet en­thu­si­asts; the Breguet ar­chives list nine watches sold to the brothers.) No. 2516 had a moon­phase, as well as in­di­ca­tions for the date, month, and day of the week, all chang­ing in­stan­ta­neously at mid­night.

The tech­ni­cal evo­lu­tion of the per­pet­ual cal­en­dar be­gan to slow after World War II, and it’s not hard to un­der­stand why — al­ways ex­pen­sive and dif­fi­cult to make, they were never in par­tic­u­larly high de­mand out­side se­ri­ous col­lec­tors’ cir­cles, and in the 1970s and early 1980s it was far from clear that any­one would be in­ter­ested in a me­chan­i­cal per­pet­ual cal­en­dar ever again. They did con­tinue to be made, how­ever — Aude­mars Piguet, for ex­am­ple, came out with a per­pet­ual-cal­en­dar ver­sion of the Royal Oak as early as 1984. And, de­spite the shaky ground on which me­chan­i­cal watch­mak­ing still stood in the ’80s, it was in that decade that a ma­jor leap for­ward in per­pet­ual-cal­en­dar de­sign came — from IWC.

In 1985, IWC showed the Da Vinci wrist­watch, ref. 3750, at BaselWorld. The watch con­tained a move­ment de­vel­oped by IWC’s Kurt Klaus, and the base cal­iber was IWC’s ver­sion of the then-al­ready-ven­er­a­ble Valjoux ref. 7750, the IWC cal. 79061. But it was the Kurt Klaus-de­vel­oped mod­ule that was the big news. The per­pet­ual-cal­en­dar mod­ule was de­signed in such a way as to make it pos­si­ble to set all the cal­en­dar in­di­ca­tions sim­ply by pulling the crown out to its first po­si­tion and turn­ing it. You can, of course, set a tra­di­tional per­pet­ual cal­en­dar by turn­ing the crown and mov­ing the hands around the dial, but the Klaus per­pet­ual mod­ule al­lowed you to quick-set the per­pet­ual cal­en­dar just as you would be able to quick-set the date on an or­di­nary watch — quickly and with­out hav­ing to move the hands. No per­pet­ual cal­en­dar had ever of­fered this abil­ity be­fore.

The key to the sys­tem was the use of a rather un­usual cir­cu­lar lever that tilted on a pivot on its edge once a day when trig­gered by a drive wheel and lever in the move­ment. This was the only point of me­chan­i­cal link­age be­tween the base move­ment and the per­pet­ual- cal­en­dar plate. Be­cause the change of all the in­di­ca­tions was con­trolled by the move­ment of a sin­gle part, it was pos­si­ble to co­or­di­nate all the date in­di­ca­tions — some­thing im­pos­si­ble in a con­ven­tional per­pet­ual cal­en­dar, which is driven di­rectly by the hour wheel from which it can­not be me­chan­i­cally un­cou­pled.

The Klaus per­pet­ual mod­ule also fea­tured a full four-digit in­di­ca­tion of the year, as well as an in­di­ca­tion of the phase of the moon. As with a con­ven­tional per­pet­ual cal­en­dar, its heart was a 48-step pro­gram wheel. The only dis­ad­van­tages to the sys­tem was that it could not be set back­wards — and that it re­quired care on the part of the watch­maker ser­vic­ing or as­sem­bling it to make sure all the in­di­ca­tions were prop­erly ar­ranged be­fore clos­ing up the case, as the sys­tem doesn’t al­low for cor­rec­tors for in­di­vid­ual in­di­ca­tions (of course, that all in­di­ca­tions are linked and can be set

through the crown alone is part of the point of the de­sign in the first place.)

It is a clever, ver­sa­tile sys­tem that is still in use by IWC to­day, and over the years, the firm has used it on base cal­ibers rang­ing from the Valjoux cal. 7750 to IWC ver­sions of the ETA ref. 2892, the Frédéric Piguet cal. 953, and the Jaeger-LeCoul­tre cals. 960 and 849; the mod­ule was also used by Jaeger-LeCoul­tre in sev­eral watches, and it’s a tes­ta­ment to the ver­sa­til­ity of the mod­ule (and to the close his­toric re­la­tion­ship be­tween IWC and JaegerLeCoul­tre un­der the late Gün­ter Blüm­lein) that Jaeger-LeCoul­tre uses a vari­ant of the mod­ule right down to the present day. Both the Master Eight Days Per­pet­ual and the Master Ul­tra Thin Per­pet­ual use this mod­ule — right down to the orig­i­nal four-digit dis­play of the year. While IWC has omit­ted the four-digit date in re­cent years (the last IWC watch to use it was the Da Vinci Per­pet­ual Cal­en­dar Edi­tion Kurt Klaus), it also re­mains the only company in the Richemont Group to of­fer the mod­ule in­te­grated with a chrono­graph func­tion.

The next step for­ward was not taken un­til 1996, but it was a big one — the first per­pet­ual-cal­en­dar watch which could not only be set by the crown, but which could also be set both for­wards and back­wards. This was the Ulysse Nardin Per­pet­ual Lud­wig, which was de­signed by Dr. Lud­wig Oech­slin, and which was part of the same pe­riod of cre­ative fer­til­ity that pro­duced, be­tween 1985 and 1992, the Tril­ogy of Time astro­nom­i­cal com­pli­ca­tions. The de­sign, like the Kurt Klaus per­pet­ual cal­en­dar, was mod­u­lar, but un­like any other per­pet­ual cal­en­dar be­fore it, there was no pro­gram wheel. In­stead, the Ulysse Nardin per­pet­ual-cal­en­dar mech­a­nism uses a group of stacked pro­gram wheels, which ef­fect the change of the date.

The mech­a­nism uses a driv­ing wheel that turns one full revo­lu­tion per day. This wheel has 24 teeth. Four of the 24 — cor­re­spond­ing to the last four hours of the day — have slightly longer teeth than the other 20. Th­ese teeth en­gage the teeth of the date-change pro­gram wheel. The date-change pro­gram wheel is ac­tu­ally a stack: a bot­tom wheel with 31 teeth; one above with 24 teeth — five of which are longer than the oth­ers; one small one on that, with four teeth, with only one shorter than the oth­ers; and fi­nally, one stacked on that, also with 24 teeth, but only one longer than the oth­ers. Thus, you have a layer cake: the bot­tom layer has 31 teeth and turns once a month, and the top three lay­ers are as de­scribed. The two 24-toothed wheels turn once a year, but the four-toothed wheel turns only once ev­ery four years.

Now, here is the clever part: the four long wheels on the driv­ing wheel are of dif­fer­ent thick­nesses, to catch — or miss — dif­fer­ent lay­ers on the date-change pro­gram-wheel cake. The thinnest catches a tooth of the date-change pro­gram wheel — on the bot­tom layer — once a day, ad­vanc­ing the wheel one tooth. This ad­vances the date one day. If the month is 31 days long, then the change from “31” to “1” is no dif­fer­ent than any other day.

How­ever, on top of the base date pro­gram wheel of 31 teeth is the next layer of the cake, layer two — the smaller wheel with five teeth longer than the rest. If, at the end of the month, it’s nec­es­sary to jump from the “30” to “1,” the sec­ond longer tooth on the driv­ing wheel — which is at a level to only en­gage layer two of the pro­gram-wheel cake — also catches, and in­stead of ad­vanc­ing one tooth at the end of the day, the date pro­gram wheel ad­vances two teeth at the end of the day — jumping the date from “30” to “1.”

This takes care of ev­ery month ex­cept Fe­bru­ary. On 28 Fe­bru­ary, the re­main­ing two of the four longer teeth on the driv­ing wheel catch on the teeth on the last two lay­ers of the pro­gramwheel cake: four teeth on the driv­ing wheel pass at the end of the day on the 28th, catch­ing on four teeth on all four lay­ers of the pro­gram-wheel cake. Since the date ad­vances one day per tooth on the pro­gram wheel, hav­ing four teeth means the date jumps four days at the end of the day on 28 Fe­bru­ary to 1 March.

What about 29 Fe­bru­ary? Well, re­mem­ber that comes once ev­ery four years. Re­mem­ber one of the wheels mak­ing up a layer of the cake has four teeth, but one shorter than the oth­ers, and

turns once ev­ery four years. On the fourth year, the shorter tooth sticks out — and it’s too short to catch a tooth on the driv­ing wheel. That means, once ev­ery four years, at the end of Fe­bru­ary, only three teeth catch — and the date ad­vances only three days, from 29 Fe­bru­ary to 1March.

Though it’s com­pli­cated to ex­plain, it’s a sur­pris­ingly me­chan­i­cally sim­ple and el­e­gant so­lu­tion, and some­thing of a Gor­dian-knot so­lu­tion to the prob­lem of per­pet­ual cal­en­dars. Gone are the springs, jumpers and levers of the tra­di­tional per­pet­ual cal­en­dar, as if in a puff of smoke; the en­tire sys­tem con­sists of gears just as happy to ro­tate for­wards as back­wards, and you have a per­pet­ual cal­en­dar you can not only re­set as blithely as you like, but also travel with, as set­ting the date back a day is no longer a prob­lem. Not only that, but the watch won’t hic­cup on 2100AD, when — in a fur­ther cor­rec­tion to a cor­rec­tion — the Gre­go­rian cal­en­dar does NOT add a day to Fe­bru­ary. In­stead, you just set the watch back one day at the end of Fe­bru­ary. The only re­main­ing common el­e­ment with the clas­sic stepped pro­gram-wheel per­pet­ual cal­en­dar is the four-toothed Fe­bru­ary wheel — the lat­est in­car­na­tion of the Fe­bru­ary star wheel of the clas­sic 12-step pro­gram-wheel per­pet­ual cal­en­dar, which, in turn, harkens back to the star wheel used by Breguet on the Marie-An­toinette. Why, if you were a ro­man­tic, you might call it the Queen’s Star.

One of the most fas­ci­nat­ing re­cent de­vel­op­ments in per­pet­u­al­cal­en­dar de­sign comes from Cartier, and was in­tro­duced in the Ro­tonde de Cartier Astro­cal­endaire, which de­buted of­fi­cially at the SIHH last Jan­uary. In cer­tain re­spects, it’s con­cep­tu­ally sim­i­lar to the Ulysse Nardin de­sign: there is no con­ven­tional 11- or 48step pro­gram wheel, and the mech­a­nism is de­signed so that it can be set ei­ther for­wards or back­wards. Like the Ulysse Nardin sys­tem, the mech­a­nism used in the Astro­cal­endaire uses a drive wheel that turns one full revo­lu­tion per day, and that ad­vances a “layer cake” date pro­gram wheel. As in the Ulysse Nardin de­sign, the date pro­gram wheel is ad­vanced, at the end of the month, any­where from one to four teeth — and thus, one to four days, de­pend­ing on which month it is.

The big dif­fer­ence, how­ever, is that the Cartier date pro­gram wheel doesn’t use a sys­tem of stacked gears. In­stead, it uses a very clever sys­tem con­sist­ing of two cams with lobes on their pe­riph­eries, which ex­tend and re­tract ex­tend­able gear teeth, set on cir­cu­lar springs above the 31 teeth of the base date wheel. The cams ro­tate in such a way that, de­pend­ing on the month, any­where from one to three ex­tra teeth are ex­tended. The lower of the two cams con­trols the po­si­tions of two teeth and can ex­tend ei­ther one or both of the teeth it con­trols. The up­per of the two cams con­trols one tooth, which is ex­tended once a year at the end of the month of Fe­bru­ary. When all three ex­tra teeth are ex­tended, the date jumps from 28 Fe­bru­ary to 1 March — ex­cept once ev­ery four years, when the tooth con­trolled by the up­per cam does not ex­tend, for the change on 29 Fe­bru­ary in a leap year.

Not only can it be set ei­ther for­wards or back­wards, but it’s got no levers or jumper springs (ex­cept for one jumper for the pro­gram wheel) and the cam sys­tem en­sures that the en­tire sys­tem op­er­ates with very low en­ergy cost to the main­spring and very lit­tle fric­tion; it’s also quite flat. It’s a beau­ti­fully com­pact piece of en­gi­neer­ing and one of the most tech­ni­cally ad­vanced so­lu­tions to the prob­lem of me­chan­i­cal per­pet­ual cal­en­dars in ex­is­tence.

Se­ri­ous stu­dents of the per­pet­ual cal­en­dar have prob­a­bly been wait­ing for the other shoe to drop — that be­ing the fact that even a per­pet­ual cal­en­dar will hic­cup once in a while, that is, once ev­ery hun­dred years. The in­ser­tion of an in­ter­calary day at the end of Fe­bru­ary is ac­tu­ally not a per­fect cor­rec­tion, and in or­der to keep the cal­en­dar from drift­ing even slightly, once ev­ery cen­tury (ba­si­cally, in any year evenly di­vis­i­ble by 100) you do not add the 29th day for Fe­bru­ary, when nor­mally you would (as we men­tioned a bit ear­lier).

For most nor­mal peo­ple, this gets a big “who cares”, but for hard­core watch en­thu­si­asts, this means you ei­ther hope you have a per­pet­ual cal­en­dar that can be man­u­ally ad­justed with­out is­sues, or you have to stop your watch, wait un­til it’s 2 March, and then set it ahead. And, it gets bet­ter (or worse, maybe) — the fi­nal hur­dle in the per­pet­ual-cal­en­dar Olympics is mak­ing a watch that ad­dresses not just the four-year leap-year cy­cle, but the 400-year leap-year cy­cle, in which, ev­ery hun­dred years, you don’t add an ex­tra day to Fe­bru­ary, but on the 400th year, you do (we’re at the be­gin­ning of such a cy­cle now; it ends in 2400AD, when you should add an ex­tra day to Fe­bru­ary even though, as it’s a year evenly di­vis­i­ble by 100, you might think you shouldn’t. How wrong you’d be.) To sum up: ev­ery fourth year is a leap year, but ev­ery 100th year is a common year; and once ev­ery 400 years, it’s a leap year.

A watch that can keep track even of th­ese very ex­tended cy­cles of in­ter­calary days is called a sec­u­lar per­pet­ual cal­en­dar, and

pre­cious few of th­ese watches ex­ist. One of the most fa­mous sec­u­lar per­pet­ual cal­en­dars is Patek Philippe’s mag­num opus, the Cal­i­bre 89 pocket watch. Patek re­ceived a patent for the sec­u­lar cal­en­dar of the Cal­i­bre 89 in 1986, and it is de­scribed by Patek as “one of the slow­est mech­a­nisms in horol­ogy” — and in­deed it is.

The sec­u­lar-cal­en­dar part of the mech­a­nism con­sists of a star wheel with four points that ro­tates once ev­ery 400 years — there is a quar­ter-turn ev­ery cen­tury. Once ev­ery 100 years, the longer teeth of the star wheel change the po­si­tion of the date wheel at the end of Fe­bru­ary so that the date changes on the 28th — and once ev­ery 400 years, a shorter tooth passes the date lever with­out en­gag­ing it, so that the 29th is added.

Svend An­der­sen, who at one point had Franck Muller as an ap­pren­tice and Patek Philippe as a client, makes another one of the very few sec­u­lar per­pet­ual-cal­en­dar watches in ex­is­tence. An­der­sen takes a some­what un­usual ap­proach in build­ing his sec­u­lar per­pet­ual-cal­en­dar mech­a­nism on an ETA ref. 2892 base, but it’s cer­tainly (well, we’re almost cer­tain) the most af­ford­able sec­u­lar per­pet­ual cal­en­dar out there. Es­pe­cially since your only other choice (un­less you hap­pen to be in the mar­ket for a Cal­i­bre 89) is from Franck Muller — the Ae­ter­ni­tas Mega 4. This ul­tra­com­pli­cated watch uses a patented mech­a­nism for the sec­u­lar cal­en­dar, con­sist­ing of a sec­ond pro­gram wheel (rather than a star wheel) that en­codes the full 400-year cy­cle for the month of Fe­bru­ary. Along with the main per­pet­ual-cal­en­dar pro­gram wheel, it en­sures that at no point will ei­ther you, or your de­scen­dants, need to ex­ert your­selves to man­u­ally cor­rect the date.

Although the Gre­go­rian cal­en­dar is over­whelm­ingly the one around which per­pet­ual cal­en­dars are built, there are many other cal­en­dar sys­tems and we feel the sub­ject of per­pet­ual cal­en­dars shouldn’t be closed with­out men­tion­ing one of the most in­ter­est­ing cal­en­dar wrist­watches ever made: the Blanc­pain Tra­di­tional Chi­nese Cal­en­dar watch.

It’s not a per­pet­ual cal­en­dar in the or­di­nary sense of the word, but it does some­thing no other wrist­watch does: show the Gre­go­rian cal­en­dar along with in­di­ca­tions for the tra­di­tional Chi­nese lu­niso­lar cal­en­dar. The Chi­nese cal­en­dar is based on a suc­ces­sion of lu­nar months of ei­ther 29 or 30 days’ length, which yields a lu­nar year about 11 days shorter than a trop­i­cal year; for this rea­son, an in­ter­calary month is oc­ca­sion­ally in­serted (this is the rea­son for the vary­ing date of the Chi­nese New Year). The Chi­nese cal­en­dar also has a “12-dou­ble-hour” day (cor­re­spond­ing to the Gre­go­rian 24-hour day). The Blanc­pain Tra­di­tional Chi­nese Cal­en­dar watch thus has two hour hands, and shows the cor­rect date and month for the Chi­nese cal­en­dar, as well as hav­ing a leap-month in­di­ca­tion and show­ing which sign of the Chi­nese zo­diac cor­re­sponds to any given year. This re­quires some 434 parts in all, with the move­ment of­fer­ing a seven-day power re­serve. The watch also fea­tures Blanc­pain’s in­no­va­tive un­der­the-lug cor­rec­tors, which al­low one to cor­rect the cal­en­dar with­out re­sort­ing to the in­con­ve­nient case­band cor­rec­tors nec­es­sary in many other watches.

Though per­pet­ual cal­en­dars have tra­di­tion­ally been de­mand­ing to build, and are there­fore rare and rel­a­tively costly, they have re­cently started to be­come avail­able more widely; such watches as Jaeger-LeCoul­tre’s Master Ul­tra Thin Per­pet­ual and, most re­cently, Mont­blanc’s Meis­ter­stück Her­itage Per­pet­ual Cal­en­dar, have made the com­pli­ca­tion more widely avail­able.

To­day, there’s an ever-ex­pand­ing range of choices and prices for lovers of this com­pli­ca­tion, and what­ever your tastes, it’s more in­ter­est­ing now than at per­haps any other time in the his­tory of watch­mak­ing to choose a time­piece that will save you the ter­ri­ble bur­den of touch­ing the crown of your watch at the end of Fe­bru­ary.

ANTI- CLOCK­WISE FROM LEFT The first known wrist­watch per­pet­ual cal­en­dar, by Breguet; the Breguet no. 160, the ‘ Marie-An­toinette’, whose com­plex per­pet­ual-cal­en­dar mech­a­nism was

de­scribed by Dr. George Daniels, above, in TheArtofBreguet

ABOVE Patek Philippe cal. 240 Q (above), a mod­ern self-wind­ing per­pet­ual- cal­en­dar move­ment LEFT The per­pet­ual-cal­en­dar mech­a­nism with its 48 steps for each month of a full four-year leap-year cy­cle

01 The Grand Com­pli­ca­tion wrist­watch by A. Lange & Söhne; 02 The Valjoux cal. 7750, on which the IWC per­pet­ual-cal­en­dar chrono­graph cal. 79061 is based 03 Kurt Klaus, in­ven­tor of the IWC co­or­di­nated per­pet­u­al­cal­en­dar mod­ule

The IWC Nove­cento per­pet­ual cal­en­dar, fea­tur­ing the Kurt Klaus-de­signed per­pet­ual- cal­en­dar mod­ule;

be­low, the Ulysse Nardin Per­pet­ual Lud­wig

The Mont­blanc Meis­ter­stück Her­itage Per­pet­ual Cal­en­dar

Only a hand­ful of sec­u­lar per­pet­ual cal­en­dar watches

ex­ist; th­ese in­clude: ( from left) the Franck Muller Ae­ter­ni­tas Mega 4, the Patek Philippe Cal­i­bre 89 and

the Svend An­der­sen Per­pet­ual Sec­u­lar Cal­en­dar

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