Here it says that Achazyahu became king in Yehoram ben Achav's 11th year. Above (8:25), it says in his 12th year!
Rashi: You are forced to say that Achazyahu's father (Yehoram ben Yehoshafat) did not die until the 12th year of Yehoram ben Achav, for he became king in Yehoram's fifth year, and ruled for eight years. Here it says the 11th year, for he ruled for one year in his father's lifetime, for his father was stricken with severe illnesses - "Negafo Hashem b'Me'av la'Chali l'Ein Marpei" (Divrei ha'Yamim II, 21:18).
Radak: Refer to 9:29:2:1 and the notes there.
Yehoram ben Yehoshafat became king in the fifth year of Yehoram ben Achav, and he ruled for eight years. If so, his son Achazyahu should have become king in the 13th year of Yehoram ben Achav!
According to Rashi, our verse discusses when Achazyahu first became king in his father's seventh year. We must say that one incomplete year was not counted. E.g. Yehoram ben Achav became king in Nisan 3045. In Nisan 3049 (his fifth year - refer to 9:29:2:1*), Yehoram ben Yehoshafat became king; Achazyahu became king in Nisan 3055, which is called his father's seventh year, and Yehoram ben Achav's 11th year. (PF)
Rosh Hashanah 2a: Nisan 1 is Rosh Hashanah for kings; we then increment the year of the king's reign, even if he became king only yesterday!
E.g. Yehoram ben Achav became king in Adar 3045. In Nisan 3048 (which is called his fifth year), Yehoram ben Yehoshafat became king, and died in Shevat in 3056, which is called his eighth year. (It also happens to be almost eight years from day to day.) Here it says Yehoram ben Achav's 11th year, for he did not yet complete 11 years of kingship from day to day. Above it is called his 12th year, based on how the king's years are normally counted. (PF)
Why does it say when he became king? Above (8:25-26) it already taught the start and end of his kingship!
Radak: Here it teaches that due to his sins, his kingship was shortened and he ruled only one year after his father.
Malbim: This teaches that not all wanted him to be their king 1 . Divrei ha'Yamim II, 22:1 says that Yoshvei Yerushalayim made him king; the rest of Yehudah did not desire him. This is why his army, which was in Ramos Gil'ad, did not come to save him.
According to Rashi, via the contradiction in the count of his years, we infer that he ruled for one year in his father's lifetime. (PF)
I do not understand how we learn from here.
