36 Hours (1965)
The Nazis capture an American Major working in intelligence and convince him it's 1950 in an elaborate plot to extract information about the D-Day invasion which German high command expects to happen in a few days time at Pas de Calais.
This feels almost Hitchcockian in it's intricacies showing the lengths the Germans would go to in order to gain vital intelligence. James Garner convinces as the captured Major and has good support from Robert Taylor and Eva Marie Saint.
Perhaps parts are a bit implausible but on the whole this was an excellent piece of war time entertainment.
There's a lengthier write up of this film in the War Films thread.
War Films