The essential difference appears to be that humans can pinpoint when something happened in the past by relating it to other events. For example, we remember our wedding day as well as who attended, what songs were played, and the happiness we felt. Dogs, on the other hand, can only distinguish how much time has passed since an event has occurred (e.g., "My food bowl has been empty for six hours."). Of course, they don't need only memory to tell them this; a growling stomach says it all.