“And it came to pass after forty years, that Absalom said unto the king, I pray thee, let me go and pay my vow, which I have vowed unto the LORD, in Hebron.”
King James Version (KJV)
15:7 After forty years - From the change of the government, into a monarchy, which was about ten years before David began to reign. So this fell out about the thirtieth year of his reign.