“They say that time changes things, but you actually have to change them yourself.”
-Andy Warhol

“Time is a companion that goes with us on a journey. It reminds us to cherish each moment, because it will never come again. What we leave behind is not as important as how we have lived.”
-Jean-Luc Picard

“Lost, yesterday, somewhere between Sunrise and Sunset, two golden hours, each set with sixty diamond minutes. No reward is offered, for they are gone forever.”
-Horace Mann

and, one of my favorite quotes ever:

“Time is the coin of your life. It is the only coin you have, and only you can determine how it will be spent. Be careful lest you let other people spend it for you.”
-Carl Sandburg