I was surprised by how much the dialogue was identical to the book, and even where it differed("Why am I out?"), there are deleted scenes to match the book. I'm not sure if this is in the book, and I would be shocked if it were, but Tom Hagen saying 'this is almost 1946,' doesn't seem reasonable in any context. The only purpose I see is to tell the audience that World War II has just ended.

The context of this statement is indeed that WWII is over, because Tom is talking about the prospects for a possible war among the families over the drug business Sollozzo is planning. What Tom is saying is that things have changed since the pre WWII days, and that the nation as a whole is war weary, and that this would not be a good time for a mob war to break out.

Yeah, but everyone in that conversation knows what year it is. Tom shouldn't be saying that to them. That is a line that works when directed at the audience, but not to the people in the room. Do you think Dick Cheney would say to someone, 'this is almost 2006. People don't want another war.'

I'm sure it was said for audience consumption. But, we say things like that all the time in different contexts. For example, you might say to someone "What's wrong with you? You're almost thirty years old." Or, "you're a married man." I think it was in part his way of emphasizing that as time passes, people generally expect things to change for the better.