'scholar' said:
The plain reading of Zechariah 1:12 is that the seventy years was a past experience of judgement and desolation brought now to the immediate attention of Zechariah with the reassurance that the temple which had lain waste for seventy years would be rebuilt.
So let's have a look at the plain reading of the verse:
So the angel of Jehovah answered and said: “O Jehovah of armies, how long will you yourself not show mercy to Jerusalem and to the cities of Judah, whom you have denounced these seventy years?
The plain reading of the English word "these" is in reference to things which are present (though 'this' could also be used in referring to the group of years as a single period); referring to a time period in such a way suggests that the speaker is either in the midst of the time period, or right at the end of the period. Since the question "how long" is one asked in frustration of the present circumstances rather than asking to enumerate the period it strengthens the position that 1) the period refers to the present, and 2) the period is still causing frustration, and is therefore not yet completed. The logical conclusion is then, that the 70 years being discussed ran from the temple's destruction in 587 until 517, during which 'indignation was shown' (Hebrew za`am - translated 'denounced' in the verse) toward the still uncompleted temple.
In addition, the angel is responding after it is indicated that "the whole earth is sitting still and having no disturbance" (verse 11), and after, goes on to say that "the nations ... are at ease" (verse 15). This further indicates that the surrounding nations were no longer under Babylon's subjection for the period discussed, further indicating that the period is not referring to the period of Babylonian subjugation.