Title pretty much says it all, but do I need to create my game between August 31st and September 15th, or can I work on it now and just submit it then?

It should be between august 21st & 15th really.
You could do what I am doing and practice before hand (but not submit that project & start a new one!)

https://twitter.com/uk_resistant

Important Jam update here: https://itch.io/jam/gbjam2018/topic/272198/this-gameboy-jam-is-cancelled