The prophet Jeremiah said that the Jews would spend 70 years in exile before the Lord would permit them to return to Jerusalem. Inquiring minds in our BSD Shepherd group looked at the dates for the exile and the return and found that they didn't add up to 70 years.
So, they asked, how do you figure the 70 years of exile? Good question! Scholars do not agree on this point--surprise, surprise.
Some commentaries say that the number 70 was not meant to be literal but was symbolic of a long time or several generations. Others quibble about the dating of these long-ago events and come up with dates that fit the 70 year prophesy but are not the same as the generally accepted dates of historians of the ancient Middle East.
Jim Taylor found an explanation that calculates the exile from the an earlier date when Nebuchadnezzar took the first captives, including the prophet Daniel, several years before the fall of Jerusalem. According to this theory 70 years elapsed between that time and the first return under King Cyrus of Persia, once the different calendar systems in use at the time were reconciled. For those of you who would like to get in the weeds of this theory, here is the web page with the details.
There is a simpler explanation that I favor and found in the Archaeological Study Bible. If you take the date of the exile after the fall of Jerusalem (586 BC) and the date that the altar was restored and the sacrifices and worship resumed under the Mosaic Laws (516 BC), you have the 70 years of exile. I like this explanation because it makes sense to me that exile is not over and restoration complete until the altar was rebuilt and worship taking place. Also it meets the KISS principle test!