Nothing is ours, except time:

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 lie 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 today’s task, and you will not need to depend so much upon tomorrow’s. While we are postponing, life speeds by. Nothing, Lucilius, is ours, except time. We were entrusted by nature with the ownership of this single thing, so fleeting and slippery that anyone who will can oust us from possession. What fools these mortals be! They allow the cheapest and most useless things, which can easily be replaced, to be charged in the reckoning, after they have acquired them; but they never regard themselves as in debt when they have received some of that precious commodity—time! And yet time is the one loan which even a grateful recipient cannot repay.

Source: Letters of Note