The Time Of Your Life

"[G]ather and save your time, which till lately has been forced from you, or filched away, or has merely slipped from your hands. Make yourself believe the truth of my words, that certain moments are torn from us, that some are gently removed, and that others glide beyond our reach. The most disgraceful kind of loss, however, is that due to carelessness. Furthermore, if you will pay close heed to the problem, you will find that the largest portion of our life passes while we are doing ill, a goodly share while we are doing nothing, and the whole while we are doing that which is not to the purpose. What man can you show me who places any value on his time, who reckons the worth of each day, who understands that he is dying daily? For we are mistaken when we look forward to death; the major portion of death has already passed. Whatever years be behind us are in death’s hands. Therefore, Lucilius, do as you write me that you are doing: hold every hour in your grasp. Lay hold of to-day’s task, and you will not need to depend so much upon to-morrow’s. While we are postponing, life speeds by. Nothing, Lucilius, is ours, except time."

(Seneca, Moral Letters, 1 "On Saving Time")

Or simply:

Don't waste it.