I think it comes down to novelty.

When I'm a regular working stiff, the months seem to fly by because each day is so similar to the last. They all just blend together in your memory, and you find yourself looking back and thinking, "Where did last month go?"

Humdrum days are also forgettable and you don't really think about them as you go through the motions. So at the end of the day, there isn't a lot of substance that your mind holds on to. Looking back, it's like they were over in an instant.

I remember when I was traveling, some weeks seemed to go on forever, because every day was crammed full of new, interesting experiences. But as soon as I got into a routine, time seemed to speed up again and the days started to run into each other.