The comment about using the AC line frequency is pretty sharp - The Hammond Clock company used to sell AC powered clocks with synchronous motors, and they would make gifts of these clocks to operators of the power utilities. The power plant operators would then have an incentive to regulate the AC line frequency.
I think in your case, you should look into a real-time clock IC. There are many of these to choose from, and you can interface to them using a variety of techniques. As has been stated in this thread, trying to use the counter in a PIC to keep accurate time in the long term is a waste of, um, time.
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