Congratulations! You've discovered instant Nostalgia!
Just kidding. You may be really coming into awareness for the first time about the nature of time passing and human mortality. Or maybe; going in the opposite direction; you're developing an obsessive/compulsive ritual..
Or maybe it's just some mental exercise because you're bored..Who knows?
No matter what though, don't let it bug the Hell out of you..Abstract thoughts are just that. Not too important to your overall life..