Circadian rhythms are a well-known phenomenon when it comes to people, but most folks may not be aware that all animals actually possess a circadian rhythm. Circadian rhythms refer to physiological, mental and behavioural responses to changes in the amount of light in the environment. These play out across a 24 hour cycle, meaning dogs would definitely respond to day/night cycles at an instinctive level, if not at a conscious level. This could explain how dogs may know when it's breakfast or dinner time, or maybe time for that afternoon walk.
My Dog Knows When I'm Coming Home!
Many pet owners claim that their dog knows what time they are due to arrive at home from work. They report that their dog waits at the door around the time they normally come home, and some even claim their dog meets them at the train station! There is no real scientific evidence that dogs have some kind of sixth sense surrounding their owners. There are many possible explanations but the most likely ones are that dogs are highly sensitive to your routines, as well as environmental and physical cues surrounding your comings and goings every day.
Do Dogs Know How Much Time Has Passed?
There have been many stories surrounding dogs and their not being able to tell how long their owners have left them alone for. A lot of these are simply anecdotes and thus not really the best evidence. There have been studies performed measuring the intensity and level of enthusiasm of dogs' greeting their owners after varying lengths of absence. There were significant differences between 30 minutes and 2 hours, yet little difference between 2 hours and 4 hours. This implies that dogs differentiate between short absences and long absences, but not with the degree of complexity that we do.