If you use an AC opto isolator with a transistor output (found H11AA3 on Digikey which should work) with a pull up resistor on the transistor output (~10k for a guess) you should be able to connect it directly to a digital Capture compare input to time the length between crossings. This circuit should output a short negative pulse every zero crossing.
I think your pic has a capture compare module that is perfect for this kind of timing.
Do you have access to the secondary of your power supply transformer (DC wall bricks won't work) ?. If so, a resistive divider or resistor/zener combination & a schmitt trigger can provide the 60Hz timing pulses. For timing, it's not necessary to have a true zero-crossing detector, you just need a circuit that gives you a consistent pulse on every cycle (duty cycle doesn't matter).