The IBM Office Products Division was, as the name suggests, the division of IBM responsible for producing products aimed at corporate use. It was established in October 1955 as the Electric Typewriter Division, later changing their name to Office Products Division as the products they were developing and marketing diversified. Notable products marketed during their tenure include the Selectric II and III "golfball"-whiffletree electric typewriters, early Selectric-based discrete keyboards, Beamspring keyboards, Model F keyboards, Model M keyboards, and the Model M-based Wheelwriter daisy wheel electric typewriters. Quite the pedigree, eh?
Perhaps a usually overlooked aspect of their tenure was the shop date system they used since the name change occurred in 1964. It is believed to correspond to US working days, with shop date 0001 likely being 1st January 1965 as that was the first operational year of the division. Shop dates were discontinued around summer 1990 as the then-named IBM Information Products Corporation was due to the be spun off into Lexmark.
Naturally, I thought it might be cool to make some sort of calculator to handle converting between actual dates and shop dates. This calculator is a prototype based on approximated variables, thus please expect inaccuracies of up to +/- 5%. Currently, an average working days per year value is being used and leap years are not taken into account yet.