Historically, the first nuclear explosive was detonated in in July of 1945.
Beyond the immediate impact of peace - the lack of increased funding comes to mind as an immediate consequence - there are other details that would change from a lack of WWII or, generally, a large-scale European conflict. If we go further, and assume that no immediately threatening government comes to power in Germany as well, how does that change the timetable?
Would the development of nuclear weapons be set back by a few years? A decade? Longer?