The global conflict, in which Frank Bryce fought and was injured, ends when Japan surrenders after atomic bombs are dropped on Hiroshima and Nagaski. The official date of the end of the war is September 2, 1945, when the document of surrender is signed aboard the USS Missouri in Tokyo harbor.
According to Rowling, it is not a coincidence that this event in the Muggle world occurs the same year as Albus Dumbledore's defeat of Gellert Grindelwald, whose revolutionary campaign parallels World War II:
It amuses me to make allusions to things that were happening in the Muggle world, so my feeling would be that while there's a global Muggle war going on, there's also a global wizarding war going on (TLC).
From the Web
Magical Mondays: World War II in Fantasy Fiction by Lady Saika