The eight years of Jehoram, son of Jehoshaphat, include it seems, the time he reigned with his father, who established his other sons out, but made Jehoram king. He seems to have reigned two years and over with his father, five or six after his death.
Jehoram, son of Ahab, began in Jehoshaphat's eighteenth year in the twenty-third year of Jehoshaphat's reign, or in the fifth year of the reign of Jehoram son of Ahab, Jehoram the son of Jehoshaphat began to reign; six years after Jehu kills Jehoram. Their years do not tally, so that the fifth may be the end, and eleven and over be counted as twelve; in the twenty-third of Jehoshaphat, Jehoram be already in his sixth, but in sum the eight years include his co-reign with Jehoshaphat.
It is said that in the Mesopotamian chronology, “begin to reign" is not counted in the dates which begin with the new year. It is also said that in Palestine part of a year is reckoned a year; thus if one reigned three months more than eleven, it is counted twelve. So that if one were short enough a year might be counted to three kings.