Charles; I'm not sure if you read this or not but Bowles goes into some detail as to how he determined that and other times: http://www.jfk-online.com/bowles.html
See chapter/section 3 in particular.
Thanks for the pointer. So Bowles derives his "approximate to the second" timestamps only during the period of time that the motorcycle radio mic was stuck on (thus causing the sound-activated channel 1 recorder to record continuously) by arbitrarily assigning the exact time of 12:29:10 p.m. (Channel 1 time) to the beginning of the stuck-mic episode. Unfortunately this doesn't help us determine anything about the time period surrounding Tippit's shooting.
Bowles also gives a number of caveats:
"It is, however, important to remember that
1. No exact record of "time" exists;
2. The several clocks were not synchronized;
3. The radio operators were not exact with regard to "time statements" on either radio;
4. The recordings were continuous only on Channel 1, and only while the mike was stuck open;
5. For an accurate, although derived, time reference point, 12:29:10 (Channel 1), the time the mike stuck open, will be developed and used in this text."