'Reign' in the Bible

In the eighth month of the second year [of the reign] of Darius [the king of Persia], the word of the Lord came to Zechariah (the Lord remembers) the son of Berechiah, the son of Iddo, the prophet, saying,

On the twenty-fourth day of the eleventh month (Feb 15, 519 b.c.), which is the month of Shebat, in the second year of [the reign of] Darius, the word of the Lord came to Zechariah the prophet, the son of Berechiah, the son of Iddo, as follows:

In the fourth year of [the reign of] King Darius [of Persia], the word of the Lord came to Zechariah on the fourth day of the ninth month, which is Chislev (Dec 7, 518 b.c.).