Time is not ours. Seneca, Epistles 4.32.3-4

Seneca advises Lucilius to find a moment of peace in which to renounce his mortal possessions voluntarily. If we cannot learn to surrender deliberately to the death that awaits, we will not be at peace with the world, no matter how we live, or where. The peace of resignation—of dedication to the best effort we can actually make, and then release—will not come to those who refuse to seek it.

Propera ergo, Lucili carissime, et cogita quantum additurus celeritati fueris, si a tergo hostis instaret, si equitem adventare suspicareris ac fugientium premere vestigia. Fit hoc, premeris: accelera et evade, perduc te in tutum et subinde considera quam pulchra res sit consummare vitam ante mortem, deinde exspectare securum reliquam temporis sui partem, nihil sibi, in possessione beatae vitae positum, quae beatior non fit si longior. O quando illud videbis tempus quo scies tempus ad te non pertinere, quo tranquillus placidusque eris et crastini neglegens et in summa tui satietate!

Hasten then, my dear Lucilius, and consider how much faster you would move, if an enemy were approaching you from behind—if you knew the cavalry were advancing, marking the trail of fugitives driven in their wake. This is happening: you are marked, and driven. Make haste and get out! Take yourself off to safety, and only then stop to contemplate how lovely it is to finish living before you die, to wait out the remainder of your time at peace, secure in the knowledge that you need not trouble yourself at all about clinging to the good life, which does not become better when it is prolonged. When shall you witness that moment of realization, wherein you'll know that time doesn't belong to you? Then shall you become calm and tranquil, letting go of the morrow in full possession of your present self!