I don't have to measure the accuracy of my timebase, just count 60 pulses to give one second ticks. The power company makes small frequency adjustments over a 24 hour time so it always averages to 60Hz. Some quick testing with a transformer and a 4N25 optic isolator gave me a decent 60Hz square wave output. Not a 50% duty cycle but more like 60% on 40% off. Setting TMR0 to 195 and then having an interrupt when it rolls over looks like it will work.