Hello Darren,

Can I get recomendations for the most accurate electronic timer that I
can buy?

It must to be accurate to within 1/60th of a second over the course of
6 hours.

This lets me assume that you need some kind of electronic triggering.
1/60th of a second is to precise for a finger to push a button.

Is something like this commercially available, or will I have to build
it, or have someone build it?

Check out the sports markets. The devices for swimming relay contests
may not quite offer 6 hours but equipment for marathons or triathlons
should. It has been a while since I did competition swimming but I
vaguely remember that the stuff mostly came from the major watch
companies, usually from Switzerland.

Regards, Joerg

http://www.analogconsultants.com