They probably set it in August of 1944. That way the "few more months" fits into when the war actually ended. It also matches up the change in earth years with the change in Narnian years (even though it isn't supposed to be like that).

I wrote an extensive answer on this question for the IMDB FAQ section a while ago, and i recently updated it to include information presented in the movie, but essentially there are two general theories as far as the book goes, and 1 for the movie.

You can read the whole thing at the link below. The second of the "Book Answers" was given by another user who disputed my original theory - essentially it all depends on whether you accept the "long ago in the war years" statement to be correct, and how you calculate the time gaps between the first three books.

http://www.imdb.com/title/tt0980970/faq#.2.1.11

As far as the movie goes, the closest thing to an official answer is the detailing shown on a prop car tax disc which we saw in some of the behind the scenes photos - it revealed the expiry date to be Dec 1943, meaning the movie had to be any time within the year of 1943 (assuming the guy wasn't driving around with an expired tax disc!). That theory also works with Eustace's statements about them having stayed with them for over 200 days prior (and the combined time they would of spent at boarding school after Prince Caspian), but you can read all that in the link above. Hope that helps.

I see. Nice sleuthing work, icarus. It's funny, I'd read about half that post a year ago, and didn't know you had written it.

So DT is set in summer 1943. I wonder, will SC follow the book and occur a few months later, or will they move it to an autumn 1944 setting to explain Will's growth?

And that in turn will either alter the gap of time between LB, or move the date of LB to the early fifties...

That's sensible, given that we knew the actors would look more than a year older than in PC, the problem with the parents crossing the Atlantic in wartime, and the fact that the next story doesn't require a specific date/time. If SC is set soonish after VDT (which will be fine assuming Will doesn't grow a great deal more) then they still have several years in the 1940s before the events of LB. (I suspect Will isn't going to look very much older even in 5 or 6 years, and be able to play Eustace convincingly as a 16/17 year old for LB.)

I had assumed it had been at least a year since the happenings in PC