I have been thinking of how to translate this:
"That was twenty years ago"
I think there are several ways to express this in German
Das ist (schon) 20 Jahre her
Das war 20 Jahre her
Das war vor 20 Jahren
I'm unsure about the possible subtle differences in meaning between the expressions. Are they all valid? I also find it interesting that I cannot really decide if to use the imperfect or the present tense when using the two first expressions