Hi, reik

Your scenario reminds me a "Thunderstrike distance measurer" ... ( sorry for the Tool name ... I am French ... )

so, if I understand it right ... the time between two interrupts could be many seconds ... ( 4 in your example ).

Consider a Pic Timer itself ( Timer1 ...i.e.) can count up to 0.52s @ 4Mhz clock ... so, you have to use an extra counter that will count Timer 1 Overflows.

your time will be ( 0.52s x nb of overflows ) + ( Timer1 x 8µs )
distance will be ... 330 ( or SQR ( 1.3*287*(Temp+273.15)) ... x count (s)

see Here for Timer calculations help :
http://pictimer.picbingo.com/index.php

BUT, you first have to open your pic datasheet to learn about the Timer modules and CCP module ( capture mode ) use.

Darrel's Interrupts just are the easy tool to drive the interrupts ... but it is you to configure what happens during the interrupts ...

The process is the same for pulse duration measuring, Rpm meters, shotshells speed measuring ...

Just need to know the maximum time between interrupts ...

Alain