The Rolex Datejust, introduced in 1945, is one of Rolex's oldest collections of watches. It was the world's first self-winding wristwatch with a date window that automatically changed over at midnight. The Datejust was not designed for specific sports, environments, or lifestyle activities, but was a classic and reliable wristwatch with a timeless aesthetic. The original Datejust was available in solid 18k yellow gold, but stainless steel and Rolesor versions became available in the 1950s. The reference 160x series introduced the Caliber 1560 movement, followed by the reference 160xx series in the late 1970s. The reference 162xx series introduced the Caliber 3135 movement, offering better timekeeping, reliability, and convenience. The Datejust II introduced a 41mm version in 2009, but the 41mm Datejust II did not immediately catch on. The Datejust 41 continues to be in production today, available in various metal combinations and with more dial, bezel, and bracelet options than any other Rolex watch line.