If you like Darrel's Instant Interrupts, you also might want to try his routines to measure time between events. His post for that is here

http://www.picbasic.co.uk/forum/showthread.php?t=365

If your time is going to be greater than 2^16 instruction, you may need to modify his code to count overflows.

Paul Borgmeier
Salt Lake City, Utah
USA