Charles Ellicott Commentary Zechariah 14:7

"but it shall be one day which is known unto Jehovah; not day, and not night; but it shall come to pass, that at evening time there shall be light." — Zechariah 14:7 (ASV)

One day. — That is, an extraordinary, unique day. (Compare Ezekiel 7:5: “An evil, an only (literally, one) evil, behold, is come.” See also Jeremiah 30:7.)

Not day, nor night. — This describes a kind of murky gloom, such as accompanies a sandstorm in the deserts of the East.

It shall be. — Or better, there shall be. As the darkest hour precedes the dawn, so the climax of man’s direst need is the precursor of the day-spring of God’s saving power. And so now, when at evening time they will be expecting the gross darkness of night to set in, suddenly they will be flooded with the light of God’s salvation.

This second half of Zechariah 14:7 is to Zechariah 14:6 and the first part of Zechariah 14:7 what Zechariah 14:3 is to Zechariah 14:1-2. In each case, the brightness of the Theophany dispels the darkness of despair.