I've been playing the 1914 scenario, and I set the end date to 28 years, but the end of scenario box still popped up at 1921, which is very annoying. Any ideas on how to fix this?
I got the same thing as the op, even when entering it starting a new game.
I think it is a bug which needs to be adressed.
I don't believe you can research the WWII technologies.
Well, there's no real point in playing past WWI, there's no flavor events afterwords
Did you set the date before starting the scenario?
Anyway, you can manually change the end date of the savegame. Go to Darkest Hour\Mods\Darkest Hour Full\scenarios\save games\ and find your savegame. Open it with a text editor and find the first occurrence of enddate. That's the end date of the scenario. Modify that and load the savegame.