Looks like the original calculator was announced here (back in 2012):
https://www.homebrewtalk.com/forum/threads/old-hops-aa-calculation.330373
"Brewing Techniques Magazine was published from 1993 to 1999
(link) ... In 2012, MoreBeer purchased the entire back stock of printed magazines. Those are available in the Homebrewing Books & Magazines section of MoreBeer’s website.".
A web search on the author / title lead to
https://www.coursehero.com/file/16936036/Hop-Storage/, which suggests that the article that may have been used for the calculations was published in the Jan/Feb 1994 issue of the magazine.
A careful read the of the JavaScript code and the article would confirm both the source and the accuracy of the implementation of the equations. Note that this doesn't address the accuracy of the equations in the article.
For something that's known to be newer,
https://www.masterbrewerspodcast.com/123 is worth a listen.