Above the main door is the colossal clock face with fresco portraits of four Prophets or Evangelists by Paolo Uccello (1443). This one-handed liturgical clock shows the 24 hours of the hora italica (Italian time), a period of time ending with sunset at 24 hours. This timetable was used till the 18th century. This is one of the few clocks from that time that still exist and are in working order.