מלכים ב, פרק ט״ו, פסוק ל״ג

A king assumes the royal throne in Jerusalem at twenty-five years of age. His time in power is recorded as lasting for sixteen years, but this number is not necessarily made up of full, complete years.

The timeline of his reign includes fragmented years. In the overall calculation of his rule, even a partial year was counted as an entire year [מצודת דוד].