Rata Die (RD) is a system for assigning numbers to calendar days (optionally with time of day), independent of any calendar, for the purposes of calendrical calculations. It was named (after the Latin for "fixed date") by Edward Reingold and Nachum Dershowitz for their book Calendrical Calculations. However, essentially the same system (including the same epoch) was used previously by the REXX programming language and by others.