It was named (after the Latin ablative feminine singular for "from a fixed date") by Howard Jacobson.
[1][2] 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.
with a decimal fraction is called a moment, with the function moment-from-jd taking the floating point R.D.
is a continuously-increasing fractional number, taking integer values at midnight local time.
It is defined as: Midnight local time on December 31, year 0 (1 BC) in the proleptic Gregorian calendar corresponds to Julian Date 1,721,424.5 and hence RD 0.