Once a year, on New Year's Eve, the mechanism comes to life. Its gears turn and, on the main calendar ring next to the computus – mirabile dictu – a metal tab changes position until it comes to rest next to the correct date of Easter for that year.

Schwilgué had made a model of the computus as well, which was stolen in 1945 and hasn't been seen since. However, clockmaker Frederic Klinghammer (1908-2006) who was employed by a company that at one time was responsible for the care of the clock, built a working model of the computus in the 1970s, and it's that model which is the basis for what modern information there is on how the Strasbourg cathedral computus actually works.

At this point you can understand why the trio who designed the date-of-Easter complication for Patek might have looked at each other and said, "Okay, guys, look ... let's just go with a program wheel." Modern fabrication techniques might make it possible to make a mechanical computus, based on Schwilgué's design, that would fit into a large wrist or pocket watch but my guess is that even with things like LIGA and silicon fabrication, it would be pushing it (though I'd kind of love it if someone would try). A 28 year program disk seems a reasonable compromise, even if replacing it with a disk for another 28 years probably involves non-minor surgery on the Caliber 89. The program disk is an unavoidable necessity as, if you use the current rules for calculating the date of Easter, a full cycle of Easter dates only repeats itself once every 5,700,000 years.