Thanks so much Kristin! Very helpful. I can't remember the details but here's what I've done so far and here's a video showing it.
I found a 30 minute count-up clock. Then had a heck of a time downloading a copy into iMovie. I finally found something that worked. http://perian.org/ allowed me to download the clip and save as Quicktime or .MOV if I recall.
I then thought about what you have done. Should I run the video capture non-stop during a 21 minute period of time? I decided to take a different approach. I video as I've always done, recording and pausing between portions of the race as I run to vantage points. I do this because I need to capture sometimes four races a day and don't want to have my batteries die or disk filled before I'm finished.
Important for me was to be sure I was recording when the gun fired. I needed to get the data code (time stamp). An 8AM race might actually go off at 7:57.4 AM according to my camcorder, which is my basis for time.
I also video the display clocks around the course and especially at the finishing chute. My clock displaying in my video needs to match that clock, whether or not I chose to include my clock throughout my entire final cut.
So Here's what I've done. This video shows the race clock in a Pic-In-Pic at various points.
http://www.youtube.com/watch?v=kmClVODKyY8
at the 4:20 mark you see 16:02 which is the race clock.
I might try your approach and see how much disk space a full race takes. It would save the editing of the clock (which saves me time) but would also add to the time where I have to edit out large sections of video I don't want. Hmmmm. .... Not sure if there's much difference but may try that.
Long story but I had to upgrade my equipment. My old Sony HD Cam died. I now have two batteries, large internal disk, two SD cards. Enjoying the Florida XC season.