Laudator Temporis Acti

Musonius Rufus, fragment 22 (tr. Cora E. Lutz, with her note):

It is not possible to live well today unless one thinks of it as his last.

This sentiment has been expressed by a number of writers. Cf. Horace Ep. I, 4, 13; Seneca Ep. 93, 6; Marcus Aurelius Εἰς ἑαυτὸν VII, 69.

οὐκ ἔστι τὴν ἐνεστηκυῖαν ἡμέραν καλῶς βιῶναι μὴ προθέμενον αὐτὴν ὡς ἐσχάτην.

Liddell-Scott-Jones, s.v. ἐνίστημι, sense III: "esp. in pf. part., pending, present". I would translate "It is not possible to live the present day well without treating it as the last."

Here are the parallel passages cited by Lutz.

Horace, Epistles 1.4.13 (tr. Colin Macleod):

Think of each dawn as lighting your last day.

omnem crede diem tibi diluxisse supremum.

Seneca, Letters to Lucilius 93.6 (tr. Richard M. Gummere):

For I have not planned to live up to the very last day that my greedy hopes had promised me; nay, I have looked upon every day as if it were my last.

non enim ad eum diem me aptavi, quem ultimum mihi spes avida promiserat, sed nullum non tamquam ultimum aspexi.

Marcus Aurelius, Meditations 7.69 (tr. A.S.L. Farquharson):

Perfection of character possesses this: to live each day as if the last, to be neither feverish nor apathetic, and not to act a part.

τοῦτο ἔχει ἡ τελειότης τοῦ ἤθους, τὸ πᾶσαν ἡμέραν ὡς τελευταίαν διεξάγειν καὶ μήτε σφύζειν μήτε ναρκᾶν μήτε ὑποκρίνεσθαι.

Related post: Good Advice.