If Hitler didn’t declare war on the US, we still would have been at war by the end of the month. We had carte blanche to aid the Brits, and it only would have been a matter of time before the Germans attacked one of our ships and giving us the Casus Belli.
Dunno if it would have been a month, for even though. FDR was itching to get in, and he was right about the threat, but would have had a harder time if Hitler hadn't declared war.
I was eight years old when Pearl Harbor was attcked and even at that age I could see and almost feel the white hot rage against Japan. IMO, the rage was such that if Hitler hadn't of screwed up, the feeling would have been "Japan is our problem, Germany is Europe's, so first things first".