Halakhah do Diwrej ha-jamim II 30:2

The beginning of Israel’s year is determined by the court, as it is written regarding Hezekiah “And the king consulted” (2 Chronicles 30:2). He intercalated the year on advice of the court, and the Passover that he conducted was in the first month (Nisan). There are clear proofs that the revered God accepted his decision. However, he made one small error, namely, he did not intercalate the year on the day before the first month. This is the meaning of “He intercalated Nisan in Nisan, to which the Rabbis did not concur” (Mishna, Pesahim 4:9).