Is not to-day enough? Why do I peer
Into the darkness of the day to come?
Is not to-morrow even as yesterday?
And will the day that follows change thy doom?
Few flowers grow upon thy wintry way;
And who waits for thee in that cheerless home
Whence thou hast fled, whither thou must return
Charged with the load that makes thee faint and mourn?

Percy Bysshe Shelley

http://www.poemhunter.com/poem/fragment-sufficient-unto-the-day/