Albert Barnes Commentary 2 Chronicles 36:21

"to fulfil the word of Jehovah by the mouth of Jeremiah, until the land had enjoyed its sabbaths: [for] as long as it lay desolate it kept sabbath, to fulfil threescore and ten years." — 2 Chronicles 36:21 (ASV)

The 70 years of desolation prophesied by Jeremiah began in the fourth year of Jehoiakim, 605 B.C. (Jeremiah 25:1, 12), and therefore should have ended in 536 B.C. However, the historical date of Cyrus’s capture of Babylon is 538 B.C., two years earlier. Because of this, it is commonly supposed that the Jews considered “the reign of the kingdom of Persia” to have begun two years after the capture of Babylon, following the death or replacement of “Darius the Mede.”

Alternatively, the term “seventy” can be understood as a round number, and the prophecy as adequately fulfilled by a desolation that lasted 68 years.

Until the land had enjoyed her sabbaths - Between the time of Moses and the beginning of the captivity, there were approximately 70 occasions when the Law of the sabbatical year (Leviticus 25:4–7) was violated.