While that seems like a convincing explanation, 750Hz is a rather odd value to use for a timer, and more importantly the overflow would be at 66d6h43m43s instead of the reported ~66d12h.

It is indeed the explanation: https://github.com/NVIDIA/open-gpu-kernel-modules/pull/1014

66 days 12 hours would put it at 747.5 Hz. A different report had 66 days 10 hours 16 minutes which works out to 748 Hz.

Maybe the clock was just feeling a little sluggish? /s

Wild.