When you use Timer, you have to take in account at least the OSC and the Timer Prescaler. The datasheet should explain it a little bit.

This link should also help
http://www.sxlist.com/techref/microchip/timer.htm

My PICMultiCalc should help you a little bit too.
http://www.picbasic.co.uk/forum/atta...7&d=1225550328