The historiography for World War 2 seems to point towards Hitler's invasion of Poland as the "official" start point of the war, however why is it that the Marco Polo Bridge Incident between ROC and Japan isn't considered the official start of World War 2? At a glance, it seems pretty Eurocentric to focus specifically on Germany, along with the China-Japan conflict being lumped into its own distinct conflict (Second Sino-Japanese War) compared with the war in Europe which was lumped into the broader WWII narrative, but would be curious to know if there's a logical reason for it.
Not to dissuade any further answers, but I find u/CrossyNZ's reply to When did WW2 start? enlightening.
tldr, start dates can be and are pretty arbitrary.
Even shorter tldr, to quote directly from u/CrossyNZ "Periodisation can be a real bitch."
More can always be said on this matter, so if anyone else would like to address the matter of war duration, please don't let this post stop you! For the meantime, OP, u/hellcatfighter has a post addressing just this question.