I still have an excel sheet that I wrote to perform mortgage calculations. The timestamp on the file is 10 March 1994. The latest version of excel can still open it and it works. Although it exists, it's not in use.

I suspect all the stuff I wrote back in the late 80's and early 90's has long been switched off at some point, which is kinda sad. It would be hard to run today: EBCDIC mainframe stuff and 16 bit PC stuff.