These lines are part of a poem by Robert Herrick called “To The Virgins, To Make Much of Time”. The full text of the poem is as follows:
Gather ye rosebuds while ye may,
Old Time is still a-flying:
And this same flower that smiles today
Tomorrow will be dying.
The glorious lamp of heaven, the Sun,
The higher he’s a-getting
The sooner will his race be run,
And nearer he’s to setting.
That age is best which is the first,
When youth and blood are warmer;
But being spent, the worse, and worst
Times, still succeed the former.
Then be not coy, but use your time;
And while ye may, go marry:
For having lost but once your prime,
You may forever tarry.
The sound samples in the song are from a scene in the movie Dead Poets Society, which shares the ‘carpe diem’ themes with ACOS.