Question about time tenses in movies

Hello Friends!
I've just joined. Looks like a very helpful site

OK This has been bugging me, and I'm sorry if I've posted in the wrong place, or infact come to the wrong forum.
My question is, in movies such as Sin City where the setting is in the 1950's or somewhere around then, but they use modern weapons and language, what is this called? I've been trying to find out all afternoon, but have gotten no where!
They also use this technique in the movie Romeo + Juliet (the one with Leonardo DeCaprio), except it's set in a modern setting but they use old English.

Thanks for any help! It is greatly appreciated.