You reference 1939, I am assuming, because that is when Germany invaded Poland (September 1st) and subsequently France and the Britain declared war on Germany (September 3rd). Why are these four countries, in Europe, the start of the world war? What makes these countries and aggressions more significant than what was happening in the pacific?

Japan invaded Manchuria in 1931, why would that not be the start? Japan had a full invasion of China in 1937 starting (unless you consider the start in 1931) the second Sino-Japanese war. Should this be considered separate from WWII until the US declares war on Japan on December 8th 1941 (or December 7th with the attack on Pearl Harbor)? Or would it not be considered a part of WWII until Germany declares war on the US on December 11th connecting the European and Pacific aggressive confrontations? I say aggressive confrontations because there was support for China from many foreign countries including Germany (Germany was supporting China against Japan prior to 1938), the Soviet Union, France, and the US but in terms of loans, military technology, supply chain, and more.

There could even be arguments on which day is the end of WWII, VE day or VJ day?

I am not a historian, but you did not ask for what the historically agreed upon start of WWII was (which likely doesn't exist and is debated). I only gave you some potential actions that could be reasonably argued as the start of WWII.