How could Rose & Bernard know that they were in the '70s?

I mean, the question is almost all in the title of the post: Rose & Bernard lost contacts with the others losties after the Flaming-Arrow attack on the beach. Thet started to live all by themselves, alone, and they had no contacts with anyone but Vincent. So: how could they know that they were in the '70s?

Also on Fandom

Random Wiki