I knew there was an astronomical clock that calculates leap years in a funky way. It just wasn't the one in Prague. It's the one in Strasbourg Cathedral.
The display of leap years calls for as much ingenuity as their calculation. On the large calendar ring, an open space between December 31 and January 1 bears the legend Commencemcnt de l'annee commune ("start of common year") ... Shortly before midnight on the December 31 before a leap year, a sliding flange that carries the first sixty days of the year ratchets backward by the space of one day, covering up the word commune at one end of the flange and at the same time exposing February 29 at the other end. The flange remains in that position throughout the year, then shifts forward again to cover up the twenty-ninth and reveal commune just as the following year begins.