QUESTION: The Gemara explains that the Jews were exiled from Yerushalayim seven years after Nevuchadnetzar's original conquest of Judea, during the reign of king Yehoyakim.
RASHI (DH Sheniyah) gives a historical account of the seven years from Nevuchadnetzar's original conquest of Judea to the exile. However, he seems to account for only six years. (See Chart.)
Rashi writes (based on Seder Olam, ch. 25) that Nevuchadnetzar captured Yerushalayim during the fifth year of Yehoyakim's reign (Yehoyakim reigned for eleven years), and he appointed Yehoyakim to continue ruling as his vassal king. Yehoyakim ruled in this manner for three years, and then he rebelled against Nevuchadnetzar for another three years. The rebellion ended when Nevuchadnetzar recaptured Yerushalayim and dethroned Yehoyakim. Yehoyakim died while being dragged to Bavel (Yirmeyahu 22:19; see also Rashi to Erchin 12a, DH Kivesh).
The death of Yehoyakim occurred six years after Nevuchadnetzar conquered Judea. Nevertheless, his death did not mark the beginning of the exile. The exile began at the death of his son, Yehoyachin, whom Nevuchadnetzar appointed as vassal king after the death of Yehoyakim. However, the verse (Divrei ha'Yamim II 36:9) states that Yehoyachin ruled for only three months and ten days before he and the rest of Judea were exiled. Accordingly, this accounts for only six years and three months from the time of Nevuchadnetzar's conquest until the exile. Why does the Gemara say that the exile occurred seven years after Nevuchadnetzar's conquest? (See RASHASH to Erchin 12a.)
(a) RASHI implies that Yehoyakim actually ruled for 11 years and 9 months, and not for just 11 years. Since his son reigned for the final 3 months of the twelfth year, that entire year was counted as the year of his own reign rather than as part of his father's. Accordingly, the seventh year was comprised of Yehoyakim's reign of 9 months and his son's reign of 3 months.
Alternatively, perhaps Yehoyakim ruled for exactly 11 years, and the following 9 months nobody ruled in Yerushalayim until Nevuchadnetzar appointed Yehoyachin as vassal king. (This is what the verse means when it says that Nevuchadnetzar returned to exile Yehoyachin "at the turn of the year" -- that is, one full year from the exile and death of Yehoyakim. See VILNA GA'ON in IMREI NO'AM, Rosh Hashanah 3a.)
(b) The IBN EZRA (Daniel 1:1) and RADAK (Divrei ha'Yamim II 36:6) imply that Yehoyakim was taken to Bavel immediately upon his original capture, and he remained imprisoned there for one year. After one year passed, Nevuchadnetzar returned Yehoyakim to the throne as a vassal king. This accounts for the missing seventh year. (According to this view, Nevuchadnetzar defeated Yehoyakim at the beginning of the fifth year of Yehoyakim's reign (i.e. at the end of the fourth year), and not at the end of the fifth year as Rashi explains.)