'Seventy' in the Bible

Then the angel of Yahweh replied, "O Yahweh of Armies, how long will you not have mercy on Jerusalem and on the cities of Judah, against which you have had indignation these seventy years?"

"Speak to all the people of the land, and to the priests, saying, 'When you fasted and mourned in the fifth and in the seventh month for these seventy years, did you at all fast to me, really to me?