The opening stock shot of Times Square includes a clock indicating the time is before 6:00 p.m. and the sky is dark. Later dialog indicates the first scene takes place in summertime, perhaps late June, when sunset in New York City isn't until about 8:30 p.m.
The day and date announced for the big concert near Arnie's hometown did not exist in 1956 (when several of the songs heard in the movie were originally composed) and was not a valid date until the year 1965. The most recent prior year in which that date occurred was 1948, predating rock 'n' roll itself.
The dialog indicates that Arnie Haines' home town is somewhere in New England when he discusses going home for vacation, but the train that he gets off has the logo of the Sunset Limited on three cars. The Sunset Limited was a luxury train of the Southern Pacific Railroad that ran along the California Coast.