What is the difference between "Danger time for America" and "Dangerous time for America"

There's very little difference, oristarA, and you are correct.

"Dangerous" is an adjective, so it is describing something that might or could happen. It might be a title of an essay on contemporary trends in the USA.

"Danger" is a noun, so it's already here. It reads more like a blaring headline in a tabloid newspaper.