“Each moment is the fruit of forty thousand years. The minute-winning days, like flies, buzz home to death, and every moment is a window on all time.”

Every Person
Lyrics, Shadows Collide with People (2004)

No known citation to Marx. First appears unattributed in mid-1960s logic/computing texts as an example of the difficulty of machine parsing of ambiguous statements. Google Books http://books.google.co.uk/books?client=firefox-a&lr=&as_brr=0&q=%22fruit-flies%22+%22time+flies%22+banana&btnG=Search+Books&as_drrb_is=b&as_minm_is=0&as_miny_is=1900&as_maxm_is=0&as_maxy_is=1970. The Yale Book of Quotations dates the attribution to Marx to a 9 July 1982 net.jokes post on Usenet.
Misattributed

Source: Metamorphoses

Source: The Tales of Alvin Maker, Red Prophet (1988), Chapter 17.

"The Syntax of Sorcery: An Interview with Tom Robbins" (2012) http://realitysandwich.com/150587/syntax_sorcery_interview_tom_robbins/ in Reality Sandwich.
Context: Forty-odd years ago, there was a countercultural moment, a brief, shining moment, as it were, when the eyes of a generation glimpsed the Eden beneath the veil.  However fleeting was this paradise, or however harsh has been its repression, its light nonetheless inspired a rowdy cohort of artists to carry its torch into the future. Tom Robbins is one of these unruly pioneers, and his frequently bestselling novels are so saturated in an uncontainable joie de vivre that they have remained virtually required reading throughout the years and decades since their initial publication.

1840s, Essays: First Series (1841), Self-Reliance
Context: These roses under my window make no reference to former roses or to better ones; they are for what they are; they exist with God to-day. There is no time to them. There is simply the rose; it is perfect in every moment of its existence. Before a leaf-bud has burst, its whole life acts; in the full-blown flower there is no more; in the leafless root there is no less. Its nature is satisfied, and it satisfies nature, in all moments alike. But man postpones or remembers; he does not live in the present, but with reverted eye laments the past, or, heedless of the riches that surround him, stands on tiptoe to foresee the future. He cannot be happy and strong until he too lives with nature in the present, above time.
This should be plain enough. Yet see what strong intellects dare not yet hear God himself, unless he speak the phraseology of I know not what David, or Jeremiah, or Paul. We shall not always set so great a price on a few texts, on a few lives. We are like children who repeat by rote the sentences of grandames and tutors, and, as they grow older, of the men of talents and character they chance to see, —painfully recollecting the exact words they spoke; afterwards, when they come into the point of view which those had who uttered these sayings, they understand them, and are willing to let the words go; for, at any time, they can use words as good when occasion comes. If we live truly, we shall see truly. It is as easy for the strong man to be strong, as it is for the weak to be weak. When we have new perception, we shall gladly disburden the memory of its hoarded treasures as old rubbish. When a man lives with God, his voice shall be as sweet as the murmur of the brook and the rustle of the corn.

Everything Is Illuminated (2002)