מלכים ב, פרק י״ח, פסוק ב׳

A new king assumes the throne at the age of twenty-five, beginning a long period of leadership. While historical records state that his rule lasted for twenty-nine years, this count does not represent full, complete years from start to finish. Instead, the calculation includes partial years [מצודת דוד]. In the official tally of his reign, even a fraction of a calendar year was counted as an entire year.