Time = Joy - Pain.

There is happiness and there is joy. Happiness relies on the ephemeral, while joy is happiness relying on the eternal. The absence of happiness is pain, hence they go hand in hand, while joy is everlasting. So you could say joy consist of happiness and pain. Pain seems eternal in its moment as well has happiness. However, both come and go.

So when you understand the the briefness of happiness and pain, and what joy consist of, you'll understand that time is the difference between what is ephemeral and eternal; what is forever and what seems to last forever; what stays and what comes and goes.