“The RDTSC instruction actually returns an unsigned 64-bit result; apparently, some Intel documentation claims that this value will not "wrap around" to 0 at contemporary clock rates for at least 10 years after the system was booted. (If you can keep an Intel system running for 9 years between reboots, you might consider telling Intel that the RDTSC counter wrapped around a year early; they might give you a refund. Or maybe not.)”
It's full of Gary Byers's (I think) detailed explanations. Recommended.