How many years passed between the official beginning and the end of World War 2?

Six years almost to the day. The "official" beginning date is usually said to be September 1, 1939, when Germany invaded Poland. This ignores the fact that Japan had invaded China in 1937, and Manchuria in 1931. The end came in 1945, with Germany giving up in May, and the Japanese signing surrender documents aboard the US battleship Missouri September 2, 1945 in Tokyo Bay.