I'd add something about it being heavily steeped in personal fantasy, because no one's dieselpunk era is identical to anyone else's.
I'm trying to boil it down to its barest essence.
If steampunk can be defined as "Victorian science fiction," then I'm sure we can define the core of dieselpunk in less than 1000 words.
my two cents: where does it end? its a good call, could be when the atomic era started (1945) or, aesthetically wts: ise when the atomic era/mid century modern started (53 or 54).