At the turn of the century there were hundreds of DEC machines of various flavors that were "tossed out" because DEC's new owners (Compaq I believe at that time) would not "certify" them as Y2K compliant. I ended up with a complete collection of every q-bus based MicroVAX ever produced, from the MicroVAX 1 through the VAX 4000-770. That represented a range of performance from .1 VUPs (VAX Unit of Performance) to about 100 VUPS or three decimal orders of magnitude.
The OSes were, but what about the apps in common use?
It's the same thing on ancient microcomputers: As far as I know, the C64's ROM-based OS doesn't care about the date at all, but various application software likely did, and would probably get very confused at two-digit years which began with a first digit less than 7.
