From the dates I can find, King Asa and Jehoshaphat overlapped in their reigns as kings. 873 - 870. Is that correct? And why the overlap?
Also, when Elijah was fleeing from Jezebel and went to Judah do we know for sure which king was reigning over Judah (Asa or Jehoshaphat).
Asa is generally believed to have ruled from 911 to 870 B.C. "And in the thirty-ninth year of his reign, Asa became diseased in his feet, and his malady was severe; yet in his disease he did not seek the LORD, but the physicians. So Asa rested with his fathers; he died in the forty-first year of his reign" (II Chronicles 16:12-13). "So Asa rested with his fathers, and was buried with his fathers in the City of David his father. Then Jehoshaphat his son reigned in his place" (I Kings 15:24).
Jehoshaphat then ruled from 870 to 848 B.C., but his last years were shared with his son Jehoram. We see this because in II Kings 1:17 it mentions that Jehoram, the son of Ahab, became king in Israel during Jehoram's, son of Jehoshaphat, second year of reigning. But II Kings 3:1 also states that Israel's new king took the throne during Jehoshaphat's eighteenth year of reigning.
What people suspect is that when Jehoshaphat went to war with Ahab against Ramoth-Gilead, he appointed his son Jehoram as co-regent while he was gone.
Then in II Kings 8:16, we find that Jehoram, the son of Jehoshaphat, again takes the throne in Judah during the fifth year of Jehoram's, son of Ahab, fifth year of reigning in Israel. That would be about the twenty-second year of Jehoshaphat's reign. Yet Jehoshaphat reigned twenty-five years in Judah (I Kings 22:42). Thus it appears there was a second co-regency for a few years.
Jehoram managed to get a total of eight years in as a king of Judah, though he only reigned two or three years on his own (II Kings 8:16-17). He died during the twelfth year of Jehoram's, son of Ahab, rule.
The overlapping rules, the common names between the two sets of kings (who are related from intermarriage), and even the use of nicknames (Joram is a nickname for Jehoram) cause tracking the kings of Israel and Judah to be difficult. Another difficulty comes from the fact that rounding is taking place. We are told that events take place in a certain year of a king's reign, but we don't know if it was at the beginning, middle, or end of that particular year that the event took place. Since the dates are being measured by two sets of reigns, it means we can be off apparently by a year depending on how the rounding works out.
If you look at the following table, I listed out the time markers. Items in square brackets are surmised, not actually stated in the Scriptures. If you add various numbers up, you will see that you are off by a year in a number of places, but that is not a problem because of the rounding issue.
|Event||King of Judah||King of Israel|
|Ahab becomes King in Israel (I Kings 16:29||Asa's 38th year of reigning||Omri's 22nd year of reigning|
|Asa becomes crippled from a foot disease (II Chronicles 16:12)||Asa's 39th year of reigning||[Ahab's 1st year of reigning]|
|Asa dies (II Chronicles 16:13) and Jehoshaphat begins reigning (I Kings 22:21)||Asa's 41st year of reigning||Ahab's 4th year of reigning|
|Ahab dies and Ahaziah becomes king in Israel (I Kings 16:29; 22:51)||Jehoshaphat's 17th year of reigning [Jehoram is probably made temporary co-regent]||Ahab's 22nd year of reigning|
|Ahaziah dies and Jehoram, son of Ahab, becomes king in Israel (I Kings 22:51; II Kings 1:17; 3:1)||Jehoshaphat's 18th year of reigning; Jehoram's 2nd year of reigning [as co-regent]||Ahaziah's 2nd year of reigning|
|Jehoram, son of Jehoshaphat, begins reigning in Judah (II Kings 8:16)||[Jehoshaphat's 22nd year of reigning]||Jehoram's 5th year of reigning|
|Jehoshaphat dies (I Kings 22:42)||Jehoshaphat's 25th year of reigning [and Jehoram's 5th year of reigning if you add in his earlier year plus co-regent reign]||[Jehoram's 8th year of reigning]|
|Jehoram dies and Ahaziah, son of Jehoram, becomes king in Judah (II Kings 8:17, 25)||Jehoram's 8th year of reigning||Jehoram's 12th year of reigning|
A few scholars, working in greater detail, conclude that there needs to be an overlap between Asa and Jehoshaphat's reign of about a year to make the dates work out. Personally, I don't see where such is necessary in order to match what the Scriptures state.
Since Elijah told Ahab there would be a drought that ended three and a half years later (James 5:17), which was immediately after Elijah's challenge on Mount Carmel and just before his fleeing Jezebel (I Kings 18:41). Elijah's flight would have had to have taken place, at earliest, right at the beginning of Jehoshaphat's reign, but more likely it occurred several years into Jehoshaphat's reign because the lack of rain was the result of the evil Ahab had created during his reign, so he would have needed several years to establish his reputation.