Mechanism: The Rolex 3155 was the sixth-generation Day-Date movement from Rolex. Calibre 3155 is a self-winding mechanical movement entirely developed and manufactured by Rolex (actually Aegler, but who really cares?) . It features a day and date calendar disc mechanism. Like all Rolex Perpetual movements, the 3155 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.

Caliber 3155 stands at 6.00mm tall and features 31 jewels. It was introduced in 1988. It is based on the calibre Rolex 3135, with additional day of week indication. Cal. 3155 is used in the Day-Date Ref. 118000 and remains in production as of 2018. The similar Rolex 3156 was used in the Rolex Day-Date II. The new Rolex Day-Date 40, launched in 2015, uses the next-generation Cal. 3255.

Like most modern Rolex movements, this model has a hacking capability. It is also the first Rolex movement with the double-quickset feature.

Family Notes: The Rolex Day-Date was introduced in 1956. It soon became known as the "Rolex President". It was unofficially renamed after President Eisenhower became associated with this model watch. This association was made as the watch was known to have been worn by LBJ and shared a scandalous connection to JFK. Since then it has earned the vote of a diverse array of potentates, including Warren Buffett, Jay Z, and (fictional character) Tony Soprano. In 1977, Rolex started using the new (at the time) 3055 movement for the Day-Date/President line. As of 2018, the Day-Date 40 uses the 3255 movement.