Well, exact precision regarding the passing of time isn't really embraced in the game anyways. Besides, those shadows found in Nagasaki and Hiroshima, still exist today, after 70 years. Granted they are probably preserved in some fashion, but still. Thermal Radiation will even bleach concrete, making it white, and the "shadow" would stay normal grey, so, also, I would imagine that it will take many years for the concrete to reach same colour (I live in a city that is over 1000 years old, and we have timber from back then, which can show you which side was on the sun side or not)
Also, yeah I remember that ghoul talk.
Aye, they were there as well! Totally forgotten those