A.T. Robertson Commentary Revelation 9:15

"And the four angels were loosed, that had been prepared for the hour and day and month and year, that they should kill the third part of men." — Revelation 9:15 (ASV)

Were loosed (ελυθησαν). First aorist (ingressive) passive indicative of λυω, "were let loose."

Which had been prepared (ο ητοιμασμενο). Perfect passive articular participle of ετοιμαζω, to make ready (ετοιμος), in a state of readiness prepared by God (12:6; 16:12; Matthew 25:34).

For the hour and day and month and year (εις την ωραν κα ημεραν κα μηνα κα ενιαυτον). For this use of εις with ητοιμασμενον see 2 Timothy 2:21. All preparation over, the angels are waiting for the signal to begin.

That they should kill (ινα αποκτεινωσιν). The same idiom in verse 5 about the fifth trumpet, which brought torture. This one brings death.