You’ll “start living tomorrow”? Start living today already, Postumus, you’re running out of time. Anyone with sense started living yesterday.

[Cras vives? hodie iam vivere, Postume, serum est:
Ille sapit, quisquis, Postume, vixit heri.]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]

Epigrams [Epigrammata], Book 5, epigram 58 (5.58.7-8) (AD 90) [tr. Nisbet (2015)]
    (Source)

(Source (Latin)). See a related sentiment by Martial in 1.15. Alternate translations:

Thou'lt live to morrow? -- 'tis too late to day:
Hee's wise who yesterday, I liv'd, can say.
[tr. Sherburne (1651)]

Thou'lt live tomorrow? -- this day's life's too late:
He's wise that lived before the present date.
[tr. Fletcher (1656)]

Tomorrow will I live, the fool does say;
Today itself's too late; the wise lived yesterday.
[tr. Cowley (1668), in Hay, "Appendix," ep. 59]

Today to live, ev'n that's too late I say.
The wiseman, Posthumus, liv'd Yesterday.
[tr. Oldmixon (1728)]

You will live, you say, tomorrow; it is late, Posthumus, to live today; he is wise who lived yesterday.
[tr. Amos (1858), ch. 3, ep. 46, noted as Martial Book 5, ep. 59]

You will live tomorrow: even today it is too late to begin to live. He is the wise man, Postumus, who lived yesterday.
[tr. Bohn's Classical (1859)]

To live today, Postumus, is already too late. He is wise, whoever he be, Postumus, who "lived" yesterday.
[tr. Ker (1919)]

"Tomorrow": -- nay, do not this moment delay.
The wise man is he who has lived yesterday.
[tr. Pott & Wright (1921)]

You'll live tomorrow? Now's too late, I say.
He's wise, my Postumus, who lived yesterday.
[tr. Francis & Tatum (1924), #255]

Yes, this is what wise Martial says,
Though in another way:
"It's much too late today to live!
The wise lived yesterday!"
[tr. Marcellino (1968), "To a Crass Procrastinator"]

"Tomorrow"? -- Postumus, today's too late.
The wise man, Postumus, lived yesterday.
[tr. Whigham (1987)]

Will you live tomorrow? It's already overlate, Postumus, to live today. He is wise, Postumus, who lived yesterday.
[tr. Shackleton Bailey (1993)]

Tomorrow? It’s already too late to live today:
He who lived yesterday, Postumus, he is wise.
[tr. Kline (2006)]

Forget tomorrow's teasing long delay.
To make life pleasant, dwell on yesterday.
[tr. Wills (2007)]

Will you live then? Today is late already.
He's wise who did his living yesterday.
[tr. McLean (2014)]

Believe me, wise men don’t say “I shall live to do that,”
Tomorrow’s life is too late; live today.