The Day-Date II is equipped with the calibre 3156, a selfwinding mechanical movement entirely designed and manufactured by Rolex. It features a day and date calendar disc mechanism. The 3156 is fitted with a Parachrom hairspring and high performance Paraflex shock absorbers developed by Rolex, offering greater resistance to shocks and to extreme conditions. Like all Rolex Perpetual movements, the 3156 is a certified Swiss chronometer, a designation reserved for high-precision watches that have successfully passed the Swiss Official Chronometer Testing Institute (COSC) tests. Its architecture, in common with all Oyster watch movements, makes it singularly reliable.