Augustine, Confessions 11.14
quid est ergo tempus? si nemo ex me quaerat, scio; si quaerenti explicare velim, nescio: fidenter tamen dico scire me quod, si nihil praeteriret, non esset praeteritum tempus, et si nihil adveniret, non esset futurum tempus, et si nihil esset, non esset praesens tempus.
So what is time? As long as no one asks me, I know. If someone asks me and I want to explain it to them, I don’t know. However, I can confidently say that I know that if nothing passed away, there would not be past time, and if nothing were still to come into being, there would not be future time, and if nothing were to exist, there would not be present time.