Time is a relative concept. Don't worry, we're not talking in an Einstein, speed-of-light kind of way. We mean in a practical, "time flies when you're having fun" kind of way.
Most of us feel like times when we're enjoying ourselves go by quicker, while times when we're unhappy, bored or stressed seem to drag on forever. This is why sometimes, when you're at work, you can feel like you've had the longest day of your life before you even take a lunch break.