time measuring's system

I try to constructive a system to measure the time of a runners ( in sec. and milisec.) fro point A to point B.
I will to use two infrared transmitters and receivers, connected to a chronometer. When the runner cross point A, the chronometer starts, when the runner crosses the point B, the chronometer stops, and I can get a reading the time in display.
But i am not sure how to connect the chronometer and how to start and stop it.I dont know what exactly kind of infrared transmitter and receivers to use too.The max. distance from point A to point B is 100 meters (329 foots)
would someone to give me ideas or advices.

Thank you very much in an advance!

Some light reading on building the IR beam break detectors.