"Es ist Zeit für das Abendessen" or "Es ist Zeit fürs Abendessen" is the correct version. "Das ist (jetzt) Zeit für ..." is not correct, though it's used in some dialects. The difference is the same as in English: "It is time for dinner" is correct, "That is time for dinner" is wrong. Remember you can use articles as demonstrative pronouns, in strength somewhere between personal pronouns ("es") and real demonstrative pronouns ("dieses").