If he arrived between 10 and midnight, depending on the moon, it could have been a dark night. He had from 11PM until 4PM to commit murder, remove the bodies, return to the scene, clean up and leave. I'm pretty sure that, based on the shadows from the green truck in the photo release, the photo was taken at about 4-4:30AM. I think that photo was taken after Garland had returned to the house to clean up.
Let's give him five hours from beginning to end. He arrives, he murders the three victims, one after the other, and drags them to the parking pad at the front of the house. No one sees a thing, so it must be late, closer to midnight than 10PM. He loads them onto the truck. I'm thinking 30-60 minutes, depending on how easy it is to load 200 pounds and six feet onto the truck. That takes an hour, roughly. It's 1AM. He leaves the scene, unloads the bodies, and is back at the scene to clean up and hose down, and he leaves by 4:15AM. That means about 3 hours to dispose of bodies, return to the scene, and hose down the crime scene. It's 45 minutes to Airdrie. Round trip is 1.5 hours, but there's also the time it takes to move the bodies from the truck to somewhere ... did police analyse the parking pad at his parent's home? There's also clean up time ... that could take another hour. Did he drop the bodies at his parent's property (take them out of the truck and put them somewhere that no one would notice if his parents came home early) and then head back to the scene to make sure he hosed everything down as well as possible to hide evidence?
He had from June 30 until July 4 to make the bodies disappear ... that's an awful lot of time.