A lot of great insights on this thread! I was going to say it’s because of routines. Once you get used to something, second nature makes it go by faster, which is why I try to give myself the least amount of routines.
An example I gave here on Fluther a while back was about taking a different road. Say there are two roads that lead to your house, and they both take the same amount of time to drive down. You always take Road A, but one day you take Road B. It will seem like it takes you longer to get home but when you time it, it’s the same amount of time. That’s because you’re not used to driving down it like you are Road A, where you see the same buildings and landmarks. Once you start aging, you start seeing the similar landmarks, and they become routine and expected.