'Seventy Years' in the Bible

Then the agent of LORD answered and said, O LORD of hosts, how long will thou not have mercy on Jerusalem and on the cities of Judah, against which thou have had indignation these seventy years?

Speak to all the people of the land, and to the priests, saying, When ye fasted and mourned in the fifth and in the seventh [month], even these seventy years, did ye at all fast to me, even to me?