In the so-called Italic system of timekeeping, which this clock is based on, the 24th hour of the day wasn’t midnight, but rather sunset, and the time for ringing the bells in the campanile adjacent to the Duomo was determined by the clock; at sundown the bells would toll to inform all that they should be within the city gates, which were locked after dark. The clock has had its mechanism replaced and upgraded on several occasions (notably, the original escapement was replaced with a pendulum mechanism in 1688) and the movement still in use today was made by the Florentine clockmaker Giuseppe Borgiacchi, in 1761. At that time, the clock was updated to a 12-hour dial, but 40 years ago a restoration returned the mechanism to its original time-telling format, which requires the clock’s current keepers, Lucio Bigi and Mario Mureddu, to keep up the ancient tradition of re-setting the hand once a week so that it always shows sundown at the correct time. The most recent restoration, completed in 2014, and sponsored by Officine Panerai, involved extensive cleaning and refurbishing of the mechanism to ensure its accurate and smooth operation, and guarantee its longevity.
Though most visitors to the Duomo snake through the nave in an enormous line that starts to form early each morning, it’s possible to actually go inside the walls of the Duomo and up a narrow spiral stone staircase, and see the room that’s housed the movement of the clock since the mid-1400s.