What is the significance of 480 years?
Malbim: He began the Bayis 480 years from when the Mishkan was built; they began building Bayis Sheni 480 years after this in the days of Ezra 1 . Mahari says, this was Hashgachah from Hashem.
Bayis Rishon stood for 410 years, and they began Bayis Sheni after 70 years of exile. The Vilna Gaon calculates the 480 years from the Mishkan until Bayis Rishon, based on the years of the Shoftim and kings. (PF)
What is "Chodesh Ziv"?
Rashi, Targum Yonasan: It is Iyar, in which there is Ziva (radiance) to trees. Gra, citing Yerushalmi Rosh Hashanah 1:2 - this is its actual name. The actual names of Nisan, Tishrei and Cheshvan are Aviv, Eisanim and Bul. The names that we call them now came into use during Galus Bavel.
Was this the second month of Shlomo's reign?
Rashi: It is second from Nisan, the first month. 1 "Li'Mloch Shlomo" refers back to 2 "the fourth year". And so it says in Divrei ha'Yamim II, 3:2 "ba'Chodesh ha'Sheni ba'Sheni bi'Shnas Arba l'Malchuso."
Malbim citing Rosh Hashanah 2a: Nisan is the first month for Jewish kings.
Gra: Likewise, "va'Yasigu Osam Chonim Al ha'Yam Kol Sus Rechev Pharaoh" (Shemos 14:23) means va'Yasigu Osam Kol Sus Rechev Pharaoh Chonim
Did he finish building the Beis ha'Mikdash then?
Radak: He began building it then.
