I was under the impression that so long as you began striking the target in the 1st hour, you got full credit for damage, and if you failed to begin the strike before the 1st hour ends, you give up 25% or so of the score for striking late.
I may have read the rules wrong.
I'm sure the 1st hour rule is to keep the frames a reasonable length. Otherwise, a frame could go 3-4 hours, not good if you START at 11 PM Eastern.