Tempus edax rerum
"Time, the devourer of everything." - Ovid, Metamorphoeses (43 BC - 17 AD)

"Time is a violent torrent; no sooner is a thing brought to sight than it is swept by and another takes its place." - Marcus Aurelius (AD 121 - AD 180)

"Even a god cannot change the past." - Agathon (448 BC - 400 BC)

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All things change, nothing perishes.
  [Lat., Omnia mutantur, nihil interit.] - Ovid, Metamorphoses

Catch the opportunity while it lasts, and rely not on what the morrow may bring.
 - Horace (65 BC - 8 BC)

Remember that man's life lies all within this present; as for the rest, the past is gone, the future yet unseen. - Marcus Aurelius