3 Sep 39 from the point of view of the British Commonwealth and France, but arguably the commencement date is -
7 Jul 37 - China
29 Sep 38 - Czechoslovakia
1 Sep 39 - Poland
10 May 40 - Low Countries
7(8) Dec 41 - USA
I guess it all depends of which hilltop you're sitting on, giving you a five year span to choose from.