How would you know?
By noting the stated times and the duration of messages in the minutes preceding the incident of the open microphone, I have, for practical purposes, fixed the time for the start of the five-minute open mike episode at 12:29:10 p.m. (Channel 1 time). Time statements broadcast later confirm this as a rational assumption. (See PART II, CHAPTER FIVE for technical details demonstrating this confirmation.) Since it is important to have a zero-base from which one might project future time points, a decision was necessary. In using the start of the five-minute interval, and 12:29:10 (Channel 1) as the zero-base, with subsequent time factored thereon, "time" would at least be constant if not absolutely accurate. If not absolutely accurate, time statements cannot be more than a second or two off. The reader is encouraged to reach an independent decision based on the transcriptions of the radio transmissions contained in the Appendix.
[From the appendix]:
As it would appear, since the Channel 2 dispatcher gave the 12:30 time and station check, "12:30 KKB364" a few seconds before Chief Curry's broadcast, "We're going to the hospital . . . ", the assassin's shots were fired either just before or immediately after 12:30 p.m. (Channel 2). However, that would be an inaccurate assumption.
There is a simple way to determine more accurately the approximate time the 12:30 station check was actually given.
Actually, the 12:30 station check was given more nearly at 12:31:16 (Channel 2).
If the clocks used by the dispatchers were more than a minute or two off of official time, then his time and station check at 12:30 would have been off accordingly. It wasn’t.