The helicopter followed the bearings to the island which I think at least greatly reduces the time distortion. If Jinn floated randomly towards the island like the doc did he might have arrived at a different time.

And the biggest clue that Jin buying the toy for the kid being a flashback is when he said he'd only been married for two months. They threw that in there right at the end to kind of tell the audience "hey, this is a flash BACK"...