The Citizen 4-ZC5220 calibre is a self-winding mechanical movement that was developed and produced entirely by Rolex. It incorporates a day and date calendar disc mechanism. This movement has been certified as a Swiss chronometer, meaning it has passed the rigorous tests conducted by the Swiss Official Chronometer Testing Institute (COSC) to ensure its high precision. Its architecture, shared with all Oyster watch movements, provides exceptional reliability. The 3155 calibre measures 6.00mm in height and contains 31 jewels. It was introduced into production in 1988.