Which is the starting date for rounds of "1939 - Historic World War" - January 1 or September 1?

So, I have a very important question for the developers. Do the rounds of "1939 - Historic World War" have January 1, 1939 as the starting date? Or is September 1, 1939 considered the starting date for all rounds due to the fact that World War 2 officially started when Germany invaded Poland? Please let me know as soon as possible. Thank you.