After following the life of Jehoshaphat, the king of Judah, the story turns its attention back to the northern kingdom of Israel to record the transfer of power following the death of Ahab [אברבנאל]. The timeline of this transition presents a distinct mathematical puzzle. Ahab ruled for twenty-two years, and Jehoshaphat began his reign during Ahab's fourth year. By a simple calculation, Ahab's rule should have ended during the eighteenth or nineteenth year of Jehoshaphat's reign. Yet, the timeline places the start of his son Ahaziah's rule earlier, during Jehoshaphat's seventeenth year.
To resolve this gap, the primary approach among commentators is that the biblical records do not tally exact, complete years. Instead, a king's final, partial year is counted twice, once as his own final year and again as the first year of the king who follows him [רד״ק, אברבנאל]. This method of calculation was used to align the timeline of the kings of Israel with the counting system used for the kings of Judah [אברבנאל]. Alternatively, it reflects a general lack of exactness in how the years of the northern kings were recorded [רש״י].
Another perspective suggests that the timeline only counts full years that have already passed. Under this view, the mention of the seventeenth year simply means that seventeen full years of Jehoshaphat's rule were complete. Therefore, Ahaziah actually began his reign at the very beginning of Jehoshaphat's eighteenth year [מצודת דוד, אברבנאל].
Regarding the length of Ahaziah's rule, he is recorded as reigning for two years, though there is broad agreement that these were not two full calendar years [מצודת דוד]. Records from elsewhere indicate that his successor took the throne during Jehoshaphat's eighteenth year. As a result, Ahaziah's time in power was quite brief, merely crossing from one calendar year into the next. His second, incomplete year was credited to him as a full year, while simultaneously being counted as the first year of the brother who took his place [רד״ק, אברבנאל].