Thanks to the FBI, we now know the exact time of the first shot with as much accuracy as we know the exact time of the third shot.
The FBI spliced seven frames out of the Tina Towner film to cover up a major camera jiggle, her reaction to that first shot. The jiggle analysis in The Final Truth reveals that she would have reacted exactly at the first missing frame.
Note the sharp upward jump during the missing frames, a radical aberration from her otherwise smooth-as-glass pan. Added yellow lines help show this sudden involuntary startle reaction within 0.307 of a second (the Towners' camera filmed at 22.8 frames per second). The splice is between T084 and T085. This video uses retouched versions of these frames not obscured by splicing cement.
Detailed account in The Final Truth: Solving the Mystery of the JFK Assassination.