One of my best rescue jobs involved doing this in 1999, yes that 1999. The client had shuttered their development department years before but was expecting to continue happily supporting and selling their simple enough alarm system products indefinitely. Testing revealed that come 2000 the alarms would just fire continually. Whoops. Fortunately there was one dev PC they'd decided to keep and not touch. Found the offending .c code and the corresponding offending machine code after some disassembly. A little bit of creative assembly language was required to squeeze an extra check in but really no big deal and the day was saved. I remember the client manager being ridiculously happy and grateful.