For thus saith the LORD, That after seventy years be accomplished at Babylon I will visit you, and perform my good word toward you, in causing you to return to this place.
After seventy years be accomplished at Babylon I will visit you - (note, Jeremiah 25:11-12; Daniel 9:2). This proves that the seventy years date from Jeconiah's captivity, not from the last captivity. The specification of time was to curb the impatience of the Jews, lest they should hasten before God's time.
I will ... perform my good word - promise of a return.