Dogs and Time

Day after day, I am greeted by my little chihuahuas at the door. No matter what time it is, no matter if I had just walked outside to get the mail, there they are waiting anxiously for me to come back in the door. They are always happy to see me. I recently had an emergency trip to Florida, where my dad underwent an unexpected quadruple bypass. I was so busy with my dad, spent most of my time there, but as I went back to my dad's home every night, I thought about my doggies and wondered if they would forget about me since I was gone so long. My sister said that dogs have no concept of time. I wonder, if perhaps that's true, because when I came home, they were overjoyed to see me, as if I had never left. What do you think? Do your puppies greet you happily at the door every time you come home?