Is it the Nabonidus Tablet you're referring to?
Insight on the Scriptures Volume 2:
What does the Nabonidus Chronicle actually contain?
Also called “Cyrus-Nabonidus Chronicle” and “The Annalistic Tablet of Cyrus,” this is a clay tablet fragment now kept in the British Museum. It primarily depicts the main events of the reign of Nabonidus, the last supreme monarch of Babylon, including a terse account of the fall of Babylon to the troops of Cyrus. Though it was no doubt originally from Babylon and written in Babylonian cuneiform script, scholars who have examined its script style say it may date from some time in the Seleucid period (312-65 B.C.E.), hence two centuries or more after Nabonidus’ day. It is considered almost certainly to be a copy of an earlier document. The tone of this chronicle so strongly glorifies Cyrus while presenting Nabonidus in a disparaging way that it is thought to have been the work of a Persian scribe, and in fact, it has been referred to as “Persian propaganda.” However, while such may be the case, historians feel that the circumstantial data it contains is nonetheless reliable.
In spite of the brevity of the Nabonidus Chronicle—the tablet measures about 14 cm (5.5 in.) in breadth at the widest point and about the same in length—it remains the most complete cuneiform record of the fall of Babylon available. In the third of its four columns, beginning with line 5, pertinent sections read: “[Seventeenth year:] . . . In the month of Tashritu, when Cyrus attacked the army of Akkad in Opis on the Tigris, the inhabitants of Akkad revolted, but he (Nabonidus) massacred the confused inhabitants. The 14th day, Sippar was seized without battle. Nabonidus fled. The 16th day, Gobryas (Ugbaru), the governor of Gutium and the army of Cyrus entered Babylon without battle. Afterwards Nabonidus was arrested in Babylon when he returned (there). . . . In the month of Arahshamnu, the 3rd day, Cyrus entered Babylon, green twigs were spread in front of him—the state of ‘Peace’ (sulmu) was imposed upon the city.”—Ancient Near Eastern Texts, p. 306.
It may be noted that the phrase “Seventeenth year” does not appear on the tablet, that portion of the text being damaged. This phrase is inserted by the translators because they believe that Nabonidus’ 17th regnal year was his last. So they assume that the fall of Babylon came in that year of his reign and that, if the tablet were not damaged, those words would appear in the space now damaged. Even if Nabonidus’ reign was of greater length than is generally supposed, this would not change the accepted date of 539 B.C.E. as the year of Babylon’s fall, for there are other sources pointing to that year. This factor, however, does lessen to some extent the value of the Nabonidus Chronicle.
While the year is missing, the month and day of the city’s fall, nevertheless, are on the remaining text. Using these, secular chronologers calculate the 16th day of Tashritu (Tishri) as falling on October 11, Julian calendar, and October 5, Gregorian calendar, in the year 539 B.C.E. Since this date is an accepted one, there being no evidence to the contrary, it is usable as a pivotal date in coordinating secular history with Bible history