Calculating elapsed time, how?

Hello,

i am using PIC16F876A, i am searching for a good way of calculating the elapsed time..

here is an explication:

i need a certain way (wether by PicBasic Pro direct programming or by use of a certain IC ) to give me what time has elapsed since the Pic was ON till i pressed a certain button that i put directly on one port of PIC.

any help?

thanks in advance.

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http://www.picbasic.co.uk/forum/show...=olympic+timer