Thanks tom and gideon your answers are probably best. How would one see how many frames are in the movie they filmed? The pressure mat sounds interesting but 2 of them is probably still quite expensive (more than 200 once you include a stop watch with an external trigger)
Any half-decent video editor will display the time in hours:minuteseconds:frames. GoPro Studio (free) does, as does Adobe Premier Elements (fairly cheap), plenty of other software available.