There may be rules for elk movement, but they don't follow those rules very well. I hunt mostly from a tree stand, but worked in the National Forest for a career. I have seen elk out feeding in the open with the temperature near 90* . I have had elk come to my tree stand at all times. Storms and the rut effect their movement. Predators including humans can get them moving. At some times in some areas, I think they get hungry early or are not filled through the night and move more in the day.
I prefer to sit at a waterhole over several trails, near a bedding area. Most of the elk in one area I hunt, have come in during the late afternoon and evening. At another location, I see them mostly in the morning. Since they have a huge area, most elk don not hang around one location very long. There are exceptions. My game cameras pick up elk at all times day and night.