The campaign is set beginning in late November, 1940.
In a few days from campaign start, - that is, on December 7th - the Japanese will attack Pearl Harbor, kicking off the American involvement in the Second World War.
Historical events happen on their respective dates - not early, not late, but exactly when and as they happened. Only direct player involvement can shift dates, times, and event outcomes as is appropriate for the plot, but even this is extremely rare.