We are taught at school to write letters with our address and date at the top. Thankfully the person who wrote down Zechariah's words was taught similarly, and gave us the information that locates these words quite precisely in history to 520BC, when the Persians had control over the whole region including Israel. It had been around 70 years since Israel fell to the Babylonians - the land was in foreign control, hence the date reference of Darius, a foreign king.