Follow my thinking here -
Ignore the clocks for one second, let's say they're all wrong and the FBI knows it, but they have blurred/hidden this fact for the sake of the investigation.
Taking our own personal attempts at synchronising clocks out, what do you have left?
Mr G sighting the bikes AROUND 12.20.
This we know as close to being a fact as it gets.
The alternative is, he is LYING.
We know he would be lying as his daughter has gone so far as to state she spoke to him, confirm WHEN she spoke to him, and WHERE he said he was when she spoke to him.
TG being incorrect about seeing the bikes implies that not only is he being WILFULLY INCORRECT (eg lying) about the sighting, it implies his daughter is lying/incorrect as well.
We know that TG is in no way suspect, is in fact a highly regarded member of the community.
So...his sighting is either truthful, or not truthful. It's as simple as that.
Common sense says it's truthful.