I almost want to post anon but I can't resist. When I took over my current job ten years ago, the company used a green-screen accounting system based on an emulated Wang 2200 running on SCO Open Server. That puts the actual technology in use back around 1973. This used the Niakwa Basic2c system. The system was lovingly maintained (!) by some dedicated guys in Auburndale.
Before we migrated it off it we got it running on Linux and I still have a KVM image running this system over Centos 5. The last time I booted it was in 2014, or 41 years after the Wang 2200 came out. I actually used one at Ashland (MA) High School - the second interactive computer I ever used. (The first was a PDP-8 accessed via a teletype at 110 baud from Wayland Junior High School).