Where is this info found? With the dates and such I mean?
In OWB, there's a terminal in some rubble sitting outside a testing station that says the weather control tech in the meteorological facility Ulysses was interested in was being tested mass scale in a town called Hopeville. There is a satellite map, presumably of Hopeville, in the meteorological facility. The town is now in ruins, and it's one long road ending in a crater. It's reasonable to assume that the Divide is in Hopeville.
As for the timing, Joshua Graham says that the loss of the Dam would have been much worse for Caesar if the NCR's supply lines hadn't suddenly been interrupted. He goes on to say that said supply lines ran through the Divide, and that Legionnaire scouts reported something awful happening there. Since this was during first war at the Dam, it is again reasonable to assume that the Divide happened four to five years ago.
People treat these as canon because they include very unusual specifications for dialog options and they don't contradict anything else.
Utah and New Reno, sure, but the impregnation line is only available to Couriers with the Lady Killer perk, so it's as canon as hitting on Arcade with the Confirmed Bachelor perk or knowing what a fish is with high Intelligence. It just irritates me when people try to use a line dependent on a perk to say, "Straight male Courier is canon!"