Megillah 12A

תניא נמי הכי: ועוד שנה אחרת לבבל ועמד דריוש והשלימה.

This is also taught in a baraita, as an indication that the years counted were only partial years: And when Belshazzar was killed, there was still another year left for Babylonia before the reckoning of the 70 years was completed. And then Darius arose and completed it. Although 70 years were previously counted according to Belshazzar’s count, from the exile of Jehoiakim, because the years were only partial, there was still one year left in order to complete those 70 years.

אמר רבא: אף דניאל טעה בהאי חושבנא, דכתיב: ״בשנת אחת למלכו אני דניאל בינותי בספרים״. מדקאמר ״בינותי״⁠ — מכלל דטעה.

Rava said: Daniel also erred in this calculation, as it is written: “In the 1st year of his reign, I, Daniel, meditated in the books over the number of the years, whereof the word of YHWH came to Jeremiah the prophet, that He would accomplish for the desolations of Jerusalem 70 years” (Daniel 9:2). From the fact that he said “I meditated,” a term indicating recounting and calculating, it can be inferred that he had previously erred.

מכל מקום קשו קראי אהדדי, כתיב: ״מלאות לבבל״, וכתיב: ״לחרבות ירושלם״!

The Talmud comments: In any case, the verses contradict each other with regard to how the 70 years should be calculated. In one verse it is written: “After 70 years are accomplished for Babylonia I will remember [efkod] you, and perform My good word toward you, in causing you to return to this place” (Jeremiah 29:10), which indicates that the 70 years should be counted from the Babylonian exile. And in another verse it is written: “That he would accomplish for the desolations of Jerusalem 70 years” (Daniel 9:2), indicating that the 70 years are calculated from the destruction of Jerusalem.