Maybe this one helps - Arduino Playground - StopWatchClass -
Anyone have experience or advice for using the Arduino in this way? A concern of mine: will the length of the course be an issue? Rather, will the length of wire (100ft) from each IR sensor to the Arduino cause any problems with signal attenuation?
100 ft is not impossible, shielded cat5 cable should work. Have you considered an XBEE (wireless) connection as an option?