Who wrote this Sefer?
Rashi: From here and onwards, Nechemyah wrote this Sefer.
Malbim: This is a continuation of Sefer Ezra. Sanhedrin 93a says that Nechemyah said all of Sefer Ezra, and explains why it was not called Sefer Nechemyah. Bava Basra 14b counts Ezra (including Nechemyah) as one among the 24 Seforim of Tanach. Also, if this were a separate Sefer, it would have specified the 20th year of Daryavesh! 1
Refer to Ezra 1:1:2:1*.
From when is this the 20th year?
Rashi: It is the 20th year of Daryavesh ha'Melech, who is Artachshasta. Rosh Hashanah 3b makes a Gezeirah Shavah. Also below (2:1) it says "b'Chodesh Nisan Shenas Esrim l'Artachshasta." They counted the king's years from Tishrei, therefore it was still the 20th year (now was Kislev). You cannot say that they counted his years from Nisan, and the episode below was earlier (the verses are out of order), for the verses prove that the episode below was due to what happened here.
