The beginning of the movie indicates that it takes place in 1959. There is a shot of a movie marquee showing "West Side Story," which actually opened in 1961.
(at around 1 min) The reflection of a boom mic is visible in the bedroom window as the camera follows Sissy Spacek going toward the window and moving the curtains. Acting like a mirror because of the lighting, the window first reflects the boom and then the mike itself as the rig moves out of the window's line of sight. A combination of the movements of the camera and the boom makes the reflection visible again before the camera pans away as Spacek walks away from the window.