Think to yourself that every day is your last - Horace

Think to yourself that every day is your last;
the hour to which you do not look forward

will come as a welcome surprise.

-Horace

Quintus Horatius Flaccus, (December 8, 65 BC - November 27, 8 BC), known in the English-speaking world as Horace, was the leading Roman lyric poet during the time of Augustus.
http://en.wikipedia.org/wiki/Horace