Rolex introduced the original Datejust in 1941 to celebrate the company’s 40th anniversary. It also featured the company’s waterproof Oyster case and a fluted bezel. Over the years, the Datejust movement went through subtle improvements to its features and an expansion of design options. The original Datejust was launched with a case size of 36mm. Later, however, ladies’ and mid-sized versions became available.