Why was a king who reigned a little over nine years recorded as reigning ten years in the Old Testament?
The Old Testament counted any part of a year as a complete year. Therefore, a king who reigned a little over nine years was recorded as reigning ten years, because the partial tenth year was reckoned as a whole year. This same principle runs throughout Scripture.
Les explains that chronologists struggled reconstructing Old Testament time because the biblical record uses inclusive reckoning. If a king reigned only a little over nine years, the remaining part of the tenth year was counted as a whole year, so the record says ten years. Les says any part of a day or year is considered a whole, and he applies this same principle to Christ being crucified on Friday yet stated as having been in the grave three days and three nights.