Julian day number

a continuous count of the number of astronomical days having transpired since Greenwich mean noon on January 1, 4713 B.C. (on the Julian proleptic calendar); the J. d. n. marks an epoch intended to be sufficiently far in the past as to precede the historical period (Seidelmann, 1992); the J. d. n. is central to the semantic resource for dates allowing the interpreter module of the standardization process to assign an unambiguous meaning to the date given on any calendar, or given the J. d. n. to calculate the corresponding date on any desired calendar