Yeah; this is actually a great question, and one with a surprisingly tricky answer. It flows from something that seems really simple when stated out loud but in practice gives historians a lot of trouble; history flows into and then out of time-frames. Depending on what you're interested in, the periodisation you'll need to create will differ.

But Crossy, I hear all the browsing readers cry, with the Second World War it's so simple! Before, there was a time that people with guns didn't fire them at people. Surely the Second World War is when angry people stormed across borders with tanks and murder? I humbly beg your pardon, readers, but that is only one method of periodisation - one method of breaking time down. And like I said before; periodisation depends on what you're interested in, not on some magic border in time. Alas!

So for some people; yes, the war began in 1939 with the German invasion of Poland. For those people, it was the conflagration in Europe - and purely military themes - which formed the primary axis around which the Second World War revolved. Perfectly reasonable! Often taught at American high schools and in American universities because it is so reasonable. It shows interest in the military and cultural impacts of the war on European and European-language-speaking countries. That is not a criticism. Things need to be broken down into chunks to be manageable when teaching.

But for some people - yes. The war began in 1937, with Japan's invasion of China. Again; perfectly reasonable. Some historians study South East Asia, after all - they aren't that interested in Europe. It makes sense to begin there, as the Japanese actions there create the conditions for everything else in the region, including those considered "part of World War Two" by those looking at Europe. This, as OP rightly points out, is taught often in South East Asia. It also implies two other things. The first is that the political and military activities of the Asia-Pacific region is as important a focus as the European - which is why historians outside South-East Asia are more commonly now using it. The second is that the war has certain themes that are unspooling through its undoubted military and cultural mobilizations. In this case, that the war is about different types of - for want of a better meta-term - "Ultra-Right-Wing" politics at their natural extreme.

Some historians step even further back, looking at themes of Empire, Decolonization, and Communism. Those historians start at 1914, and view both World Wars as the same - continuations of each other. This is also a perfectly reasonable period; after all, the First World War created conditions quite different from those that existed before (culturally and politically). The two wars were fought for very similar reasons and for very similar impulses; did the first world war really resolve tensions relating to Nationalism, or were those things continued into the Second? Why break them apart when they make so much more sense taken together?

Or how about those historians studying communism; some of those start the twentieth century in 1917, and end it in 1991. It actually seems reasonable to argue that communism was that century's defining feature - that the Second World War should be understood inside that period as a particular campaign as part of a wider political set of anxieties (after all - wars are fought by cultures for political purpose - can we talk about a war being in isolation from the context with created it?)

A lot of readers are going to be pointing at me and going - you missed the point. The war was from this to this time. All the historians I've read say that. Only military things should define the war! And fair enough! But most people will also be reading particular kinds of history, will have been taught particular kinds of history, and that periodisation is appropriate for both the history they are reading and the history they choose to be taught. I truly am begging you to understand the start and end dates of anything - whenever a historian says "I am going to start here in the story" - those starts are arbitrary. Most historians will start their histories of the Second World War with a chapter of context because otherwise the military stuff doesn't make any sense. How can we say it starts there if even when trying to teach it we can't start there?

So here is the broad take-away from my post. Leaving aside the question of when World War Two started, and thinking in just plain old anything terms now. Know that when you pick what time periods you're going to study, or you pick a historian with a particular set of time frames, then you need to understand that those times have been chosen to reflect the concerns and bias of the historian. You need to consider what that historian is putting in and what they are leaving out when they chose those times.

TL:DR Periodisation can be a real bitch.