Factual error: In the police station, when Dave receives a call from Jim, the phone sits atop a phone book marked "Seaford/Laurel" on the spine. The towns of Seaford and Laurel are next to each other in Maryland, while the movie takes place in Chester County, Pennsylvania, notably around Downingtown and Phoenixville. Laurel is over 100 miles from Chester County. There is no reason for a Chester County police station to have a phone book from towns 100 miles away. (04:03:33)
Continuity mistake: When everyone is running out of the theater, a woman with a red belt and yellow purse can be seen exiting with the other terrified patrons. A few seconds later, the exact same woman is seen running out of the theater again.
Other mistake: At or just after the point where Steve McQueen restrains the diner owner from running out the door - McQueen's wedding ring is visible on his left hand throughout one entire long-shot. (He was 27, despite playing a teenager, and had been married to Neile Adams for more than a year).
Other mistake: In the scene where everyone is running out of the theater most are cracking up. (01:11:00)
Visible crew/equipment: In the beginning of the movie (about 10 mins. in) Steve McQueen and Aneta Corsaut are driving along and think they hit an old man on the road. They stop the car and get out. There is a shot of the rear right panel of the car, two sets of "lights" are visible reflecting off the car. One single light and 4 lights in a square shape to the right of it. They help the man into their car and there is another side shot of the car and the lights are now 3 sets of 2 lights. They're in the middle of nowhere, at night, along a country road. It looks like "light trees" used to illuminate a scene at night.
Factual error: Steve McQueen remarks that the chunk of meteorite is hot - though this is a common misconception (and probably belongs on several movies' boards), meteorites are almost invariably cold, given that by the time they hit Earth their frigid inner layers are exposed by the friction of air drag. (00:20:40)
Other mistake: The movie supposedly takes place in the Downingtown, Pennsylvania area (although the theater is in nearby Phoenixville). On the (supposedly) Downingtown police station desk, under the phone, is a phone book for the Seaford/Laurel area which is in southern Delaware, almost 120 miles away. (00:43:53)
Answer: Because movies aren't always set in the year they're released. Absent any conflicting information, the assumption must be that the events of the movie take place around July 1957.