You can also program a "software trimmer capacitor" of sorts in your ISR...  Just add or subtract 1 from TMR2 (or whatever timer you're using) for N number of interrupt cycles each second...  You could potentially realize accuracy of within plus or minus one Tcyc per second or 86,400 Tcyc per day (0.01728 seconds when using a 20-MHz clock and 1:1 prescaler on TMR2), not including temperature drift and crystal aging...  Unfortunately, if your frequency correction factor N is too high, it would interfere with the accuracy of your 10th or 100th second 'ticks'...

Good luck with your project...  Regards, Mike
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