abstract
| - 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. Rata Die is somewhat similar to Julian Dates (JD), in that the values are plain real numbers that increase by 1 each day. The systems differ principally in that JD takes on a particular value at a particular absolute time, and is the same in all contexts, whereas RD values are relative to timezone. This makes RD more suitable for work on calendar dates, whereas JD is more suitable for work on time per se. The systems also differ trivially by having different epochs: RD is zero at midnight local time on December 31, 1 BC in the proleptic Gregorian calendar (thus January 1, AD 1, or 1.1.1 corrsponds to RD 1), whereas JD is zero at noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar.
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