Why different release dates worldwide?

This is an unfortunate hangover from times past when social media didn't exist and studios could pick and choose where and when a movie gets released.

Unfortunately the management at the top hasn't got the message yet... so yep movies should be released worldwide on the same date so spoilers won't really exist and everyone who wants to see the movie can see it with fresh eyes.

The difference between several days can be called an error.
Does everyone know an opening to the public day in Japan?
It's September 15....
A Japanese alien fan is disappointed by the grief and anger.

Yes it really is weird. Though even if they did release the movie on the same date worldwide, the plot would still be leaked via online media due to time differences. Here in Australia, it opens on the 11th. I don't understand why Japan has to wait an extra 4 days as you're only 8hrs away from us. I'm putting it down to the movie being partly filmed here.