NavList:

"And they had a neat little section way in the back explaining this seemingly miraculous new algorithm for date computations. You take the INT function of this quantity, another INT function of another quantity, maybe throw in an integer division, and, whaddaya know, the julian day number just drops right out. The time was right for the publication of this little algorithm, even in staid resources like the Explanatory Supplement (if, in fact, that decades-old bit of my memory is correct) because programmable calculators were then fairly common and home computers were beginning to take off (the Apple II computer was launched in 1977)."

It was Henry Fligel and Thomas Van Flandern who came up with the algorithm, which could be coded in one line of FORTRAN, and published it as a letter to the editor in the Communications of the ACM in 1968. Copy attached. Makes use of modulo arithmetic as does GPS System Time (and all time systems for that matter and why it's sometimes referred to as clock arithmetic in that regard). I talked about it in one of my GPS World columns years ago. Copy also attached. Henry is still with us but I last saw him about 10 years ago.