<pedantry corner>CFront was never compatible with K&R C to the best of my knowledge, so the actual start date would be whenever C89-style code in widespread use; I'm not sure how long before 1989 that was.
I can tell that during 1999 - 2003, the aC compiler we had installed on our HP-UX 11 development servers, still had issues with C89, we had #defines to use K&R C function declarations when coding on that system.
