2 Kings 15:30-3 says:
There is no contradiction between verse 30 and verse 33. "Twentieth year of Jotham" does not necessarily mean the twentieth year of his reign as king. We must not add the words "of his reign" because it does not exist in the text and does not fit the context of verse 33. The preposition "ל" in the phrase "of Jotham" (ליותם) simply relates "twentieth year" to "Jotham". Thus "twentieth year of Jotham" simply means the twentieth year after Jotham - namely, the twentieth year after Jotham came on the scene. Such an interpretation is consistent with the context and does not pose a contradiction.