1)
Why does it say "ha'Shanah ha'Hi ba'Chodesh ha'Shevi'i"? The seventh month (Tishrei) begins the next year!
1.
Rashi (from Yerushalmi Sanhedrin 11:5): This teaches that he died on Erev Rosh Hashanah, and commanded his sons and household about the matter to take him out [for burial] after Rosh Hashanah, to make it appear that Yirmeyah's prophecy was false.
2.
Radak: The Nevu'ah was that he will die within one year of the Nevu'ah. One who vows 'l'Shanah' counts 12 months from when he vowed. If he said 'l'Shanah Zu', it is only until the end of Elul, even if he vowed on Elul 29.
