Albert Barnes Commentary Ezra 6:15

"And this house was finished on the third day of the month Adar, which was in the sixth year of the reign of Darius the king." — Ezra 6:15 (ASV)

“Adar” was the twelfth or last month of the Jewish year, corresponding nearly with our March. The sixth year of Darius was 516-515 B.C.