Strolling through the web I've run into many definitions od Dieselpunk. They're not all the same, as you may suppose. It came to my awareness, that the moot point of it is the question of when the diesel era has ended. Some claim that the ending border of that setting is the begginning of the 50s, the others, that the truce in 1945 was such point. I'd like you to ask: what do you think?
Personally I find "dieselpunk" period of time to end in the late 50s. The culture, the art, the fashion of this decade also fits in the terms of the diesel era, and moreover it's enlarging it by adding, for instance, the "rebellious" new generation of teenagers, who though still retain the class and style of previous decades.
Some of the most recognisable elements of Dieselpunk culture such as Bioshock, Mafia II (maybe not exactly dieselpunk, still considered to be set in the diesel era) or upcoming The Bureau set their action in the late 50s. It's enough proof for me.
For me, it begins in 1900 (Where Steam and Diesel can mix, as Steam is ending and Diesel is becoming prevalent) and ends late 50s. While a universe I'm (Attempting to) creating borrows some from the 50s/60s, especially atomic energies, the aesthetic and attitude is more based in the 20s, 30s and 40s. America goes into a second 'Golden Age' and Art Deco makes a comeback, somewhat departing from the 50s architecture and 'raygun gothic' aesthetic as much of the industry is more military based in aesthetic, rather than the aforementioned raygun gothic space helmets and antennae attached to the head/feet/extremities.
Between the space travel and nuclear engines prevalent in some spacecraft and some Airships, the universe takes after Raygun Gothic/Atompunk for the sake of my sanity. In everything else, including clothing, art deco, jazz and other prevalent styles and the grim military of the 40s it's very Diesel.
My opinion, all genres of Punkpunk have eras of bleed in which they mix together with their predecessor/successor, and so as long as you keep to the attitude of a certain genre and show some fidelity to it, it can be called as such.
Granted, not likely to have a steampunk universe with a nuclear warhead. But that's just me.
I think there is no fixed rule. For me, dieselpunk ends with the decline of the big band swing era. Rock'n roll does not really fit to my dieselpunk feeling. In fashion, dieselpunk ends when men stopped wearing fedoras on a daily basis. So, the end of dieselpunk is not a fxed year but rather a cultural change. I would agree with most others here that dieselpunk starts after WW1.