'Thirty' in the Bible

All the commissioners of the kingdom, the prefects and the satraps, the counselors and the governors have consulted and agreed together that the king should establish a royal statute and enforce an injunction that anyone who petitions (prays to) any god or man besides you, O king, during the next thirty days, shall be thrown into the den of lions.

Then they approached and spoke before the king regarding his injunction, “Have you not signed an injunction that anyone who petitions (prays to) any god or man except you, O king, within the designated thirty days, is to be thrown into the den of lions?” The king answered, “The statement is true, in accordance with the law of the Medes and Persians, which may not be altered or revoked.”