The frescoed face of the Duomo clock is gigantic, totalling some 45 square metres. Paolo Uccello painted this masterpiece of the Early Renaissance in 1443, but it had been lost under overpainting until late in the 20th century, when the original qualities were finally rediscovered. The clock mechanism from the 18th century is housed in an interspace inside the wall: this replaces the original of the 15th century, but still retains manual winding. Prior to the restoration works, studies of archival records and of other works by Paolo Uccello, in particular the comet star in the stained-glass Nativity, led to the remaking of the clock hands in the form of a rayed star.

The large hand indicates the 24 hours of liturgical time, in a movement ‘anti-clockwise’, beginning from the hour of sunset positioned at the bottom of the dial.

Paolo Uccello painted the display in the form of a square with an inscribed circle: the hours distributed within like a corolla of petals, opening around the dark blue circle, and each indicated in Roman numerals. At the corners are four tondos, painted as cylinders in perspective: from these emerge four haloed heads, of startling naturalism, representing either four Prophets or more likely the Four Evangelists. The geometry of the quadrant recalls Brunelleschi's architectural plans designed in circles and squares, for example of the old Sacristy of San Lorenzo. A more direct reference for Uccello would have been the ancient zodiac (1207) inlaid in the marble floor of the Baptistery (or its twin in the Basilica of San Miniato): also an instrument of time, in this case a solstitial gnomon, or “sundial”.

The dominant scholarly opinion is that the heads depict the four evangelists, and that their arrangement at the four corners signifies their proclamation of the Word, reaching to the four corners of the earth. The Word, which is Christ, is symbolised by the star shapes of the hands, radiating out on the dark ground, just as is written in the first chapter of the Gospel of John: Jesus is “the light of Life that shines in the darkness.”