'Thirty' in the Bible

All the presidents of the kingdom, the deputies and the satraps, the counselors and the governors, have consulted together to establish a royal statute, and to make a strong decree, that whoever shall ask a petition of any god or man for thirty days, except of you, O king, he shall be cast into the den of lions.

Then they came near, and spoke before the king concerning the king's decree: Haven't you signed an decree, that every man who shall make petition to any god or man within thirty days, except to you, O king, shall be cast into the den of lions? The king answered, The thing is true, according to the law of the Medes and Persians, which doesn't alter.