A dog's understanding of time is more grounded in concrete terms, specifically ones that relate to the present. The easiest way to think of this is in "how long since" terms. For example, your dog has a basic understanding of how long since you left the house this morning, how long since you fed him last or how long since his last walk. He doesn't track time like humans do, but he can tell if you left just a few minutes ago or a few hours ago, and it affects his mood.