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)
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)
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.