'Seventy' in the Bible

Then the angel of the Lord, answering, said, O Lord of armies, how long will it be before you have mercy on Jerusalem and on the towns of Judah against which your wrath has been burning for seventy years?

Say to all the people of the land and to the priests, When you went without food and gave yourselves to grief in the fifth and the seventh months for these seventy years, did you ever do it because of me?