Irrevocable

"Time is the most precious gift in our possession, for it is the most irrevocable. This is what makes it so disturbing to look back upon the time which we have lost. Time lost is time when we have not lived a full human life, time unenriched by experience, creative endeavor, enjoyment, and suffering. Time lost is time not filled, time left empty."
--Dietrich Bonhoeffer