“Thus says Yahweh.
‘Only when Babylon’s seventy years
I will fulfill to you
I will bring you
Back to this place.’”
Clearly Yahweh says, via this letter of Jeremiah, that the captivity will last 70 years. 70 years from 598 BCE, the first exile, would be 528 BCE, while 70 years from the second exile in 587 BCE would 517 BCE. Yahweh definitely was going to fulfill his promise to bring them back to Jerusalem, but not right now.