Albert Barnes Commentary Zechariah 7:1

"And it came to pass in the fourth year of king Darius, that the word of Jehovah came unto Zechariah in the fourth [day] of the ninth month, even in Chislev." — Zechariah 7:1 (ASV)

In the fourth year of Darius - Two years after the series of visions, shown to him, and two years before the completion of the temple. Chisleu being December, it was the end of 518 b.c.