Humans respond to what we call a circadian rhythm where we mark passage of time over the course of the day using cues like light or darkness.
There are historical examples of timekeeping abilities in dogs. Perhaps one of the most famous is Hachiko, an Akita owned by Dr. Eisaburo Ueno, a professor at Tokyo University. Hachiko accompanied his master to the train station each day to see him off. He would then leave only to return to the station each afternoon in time to greet his master. One afternoon, Professor Ueno did not return—he had died in Tokyo. Hachiko waited at the station until midnight. The next day, and every day for nearly 10 years thereafter, Hachiko came to the Shibuya station at exactly the right time to meet the train which his master always used to arrive on.
Alexandra Horowitz, a psychologist from Barnard College in New York City, believes that she has the answer. She suggests that dogs may well be smelling time changes. As an example, she points out that many dogs can tell which way to follow a scent trail by deciding to travel from where it is weakest (oldest) to where it is strongest (most recent) even though the degree of change in scent intensity might be very tiny over the distance of a dozen or so steps. Since stronger odors are often newer and weaker ones are older, that means that when dogs smell weak odors they are perceiving events of the past.
Take your home environment, when you walk out of the door in the morning, the intensity of your scent in the house decreases with each hour that you are gone. It is possible that your dog has learned, through simple repetition, that when your odor has weakened to a specific level, this is when you usually come through the door. In other words, the strength of your residual smell in the house is what is predicting the time when you return home.