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Rata Die
Rata Die (R.D.) 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 ablative feminine singular for "from a fixed date") by Howard Jacobson.〔 The same system (including the same epoch) was used earlier, e.g., the REXX programming language since about 1980:〔(REXX/400 Reference manual ) page 87.〕
The base date of 1 January 0001 is determined by extending the current Gregorian calendar backward (365 days each year, with an extra day every year that is divisible by 4 except century years that are not divisible by 400).
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 R.D. values may be relative to timezone, depending on the implementation. This makes R.D. 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: R.D. is 1 at midnight (00:00) local time on January 1, AD 1 in the proleptic Gregorian calendar, JD is 0 at noon (12:00) Universal Time on January 1, 4713 BC in the proleptic Julian calendar, and the REXX count of days begins with 0 at midnight (00:00) local time on January 1, AD 1 in the proleptic Gregorian calendar.
==Forms==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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