Plot hole: The aliens provide coordinates for a place to make a rendezvous with humans but do not specify a time. It's possible they're monitoring and will arrive when they see enough humans gathering at Devil's Tower, but the humans seem to expect the aliens to come more or less exactly when they actually do, somehow.
Suggested correction: Scientists were prepared for the aliens and knew when and where they would arrive, as seen by the extensive complex built at Devil's Tower. However, as Claude Lacombe, the French scientist, speculates towards the end, hundreds of humans may have had the Devil's Tower image recently implanted in their minds, saying they were "invited" but never made the connection. Neery figured it out and was compelled to go there at that time. Neery was then allowed to join a group of trained volunteers that were already prepared to go on the ship, while the previous "abductees" were being returned to Earth.
The aliens had not given a time to meet. The scientists may be taking it on faith that the aliens will know that they've arrived at Devil's Tower and are ready, but the way the subject of timing is left unaddressed on screen feels like an oversight.
We don't know for certain if the scientists were given a specific time, but it appears they were, or at least a general window. The long-lost objects, like the ship and the military aircraft, suddenly showing up in the desert, is an indication the process has started. If humans were given the precise location where the alien ship would arrive (Devil's Tower), then, logically, the aliens would also communicate when. The scientists were communicating with the aliens using tonal sounds. Early on, the scientists received map coordinates through dish satellites as repeating pulses. They would likely receive time information the same way. As often happens in movies, this info may be something that got edited out of the film, causing an inconsistency.
This might be one of those edge cases. Under most circumstances I'd agree we could assume arrangements were made off-screen by virtue of the fact that the rendezvous occurs successfully in the first place; but in the context of this movie, in which any and all forms of contact with the aliens is treated as profound and significant, leaving it unaddressed (not even with a line of dialogue) comes off like a plot hole. I suppose we'll just have to let our fellow website readers decide.
Continuity mistake: Roy meets Jillian and then walks away to look after Barry. In the next shots with the kid, Roy's hair is longer and messed up.
Audio problem: In the scene just as Roy, Gillian, and Larry take off for Devil's Tower, the army officer is on his car phone speaking to his superior. When his superior tells him if they don't get Roy and company off the mountain in one hour, to use EZ4. Lacombe, who was talking to David at the time, looks at the officer and says, "What's EZ4?". Lacombe should not have known what was said over the phone, as only the officer and we, the audience, knew what was being said. It wasn't a speaker-phone. [The army officer was using a VRC(vehicle radio console.) Not a cellular phone. The radio has a speaker mounted on the front of the console in addition to the speaker in the handset. So everyone would be able to hear the conversation on the radio. ].
Continuity mistake: The mess next to the fridge differs between shots, the egg box flips over for example.
Continuity mistake: When Ronnie rescues Roy from the bathtub, a rope next to him disappears between one frame and the other.
Continuity mistake: When Roy is driving to Devil's Tower he is about to crash against two cars. From the opposite angle the cars are different.
Continuity mistake: In India, Claude asks the Guru where the sounds came from, while a huge crowd walks along the desert. When a man in blue addresses the crowd to ask them, the crowd on the right side is gone, and the ones on the left aren't walking but standing perfectly formed like an army.
Continuity mistake: In Jillian's house, when the aliens launch the floor vent it lies on the right side, half a meter away from the hole. A frame later it's lying skewed on the hole. Should the aliens have moved it, a sound would've been heard.
Continuity mistake: When Roy is shoveling dirt through the window he leaves his shovel on the wheelbarrow. When he comes back three seconds later, the shovel is in a different position.
Continuity mistake: When Roy starts throwing plants in the window, the glass gets smashed. Then they open it. When Roy closes the window after he climbs in, it is no longer smashed.
Continuity mistake: Roy's first encounter is with a ship that comes up behind, goes overhead and briefly reappears in front. When it's glimpsed passing above, it's moving towards him as if it came from ahead.
Continuity mistake: Barry is leaving his house, going towards the trees. Jillian looks out her upstairs window and Barry as well as the immediate area surrounding him are drenched in bright direct light, as if from a spotlight mounted on the side of the house. However, from an outside angle of the scene he is not bathed in light, and neither is the area he is in.
Character mistake: When Roy is heading toward Devil's Tower in his car, you can hear a radio report. The man says something about the Tetons and then mentions the town of Meteetsee. He says "Meh-test-ie." That's the incorrect pronunciation.
Continuity mistake: The pattern of shaving cream on Roy's face changes during the scene in which he gets fired.
Continuity mistake: When Barry is taken from the house, just before that happens we see a grate get blown out of the floor and it lands on its side. In the next scene the grate is back in the hole, but at an impossible angle, even if it fell in.
Continuity mistake: When Jillian's kitchen goes wild, the heater's lids fly off and reveal some red foam which ends up breaking and making a hole. A frame later from a new angle the foam is fixed.
Continuity mistake: Roy is about to run Barry over with his truck and stops on the side of the road. The position of the papers on the dashboard changes between shots.
Continuity mistake: Ronnie says "It's one of those things" and Roy follows her with his hand filled with shaving foam. In the next angle the amount of foam on the right hand is bigger.
Continuity mistake: When Roy and Jillian meet, the redness on Roy's face keeps darkening or lightening randomly between shots.
Continuity mistake: When Barry walks outside the house and his mother calls him, from the shots inside she is surrounded by curtains moved by strong air. From the opposite angle the curtains are gone.
