The Rolex Daytona reference 16520, introduced in 1988, marked a significant evolution in the Daytona line by incorporating an automatic movement. This model, often referred to as the "Zenith Daytona," features the Caliber 4030, a modified version of Zenith's El Primero movement. Rolex's refinements included reducing the beat rate from 36,000 to 28,800 vibrations per hour and removing the date function. ​