“Time flies” means that time passes quickly, but there’s a popular riddle:
 
Q: Why did the man throw a clock out the window?
A: He wanted to see time fly.
 
The joke has been cited in print since at least 1949.
 
“Why did the boy throw butter out the window?” is a similar riddle. “Time flies when you’re throwing watches” and “Time flies when you throw away calendars” are similar “time” riddles.
           
       
Old Fulton NY Post Cards
27 April 1949, Nassau Daily Review-Star (Freeport, NY), “The Cutie Club” (For Girls From 4 To 14 Only) by Lynne Carroll, pg. 20, col. 1:
Cutie enjoys reading all your letters but she’s just a bit worried—Mary Haynes, Baldwin, sent in a note with a joke.
 
Why did the moron throw the clock out the window?
He wanted to make time fly.
 
11 May 1950, The Journal-Tribune and Shopper (Williamsburg, IA), pg. 3, col. 6:
“Why did the moron throw the clock out the window?”
“He wanted to see time fly.”
 
20 July 1952, Leavenworth (KS) Times, “Riddles,” The Junior Times, pg. 13, col. 2:
Why did the little moron throw the clock out the window?
Answer: He just wanted to see time fly.
Carol Ann Kern, 13,
RR 1.

26 September 1953, Journal and GUide (Norfolk, VA),“Riddles” (Sent in by Barbara Lee Bunker, Mount AIry, N. C.), pg. 17, col. 3:
Why did the little moron throw his clock downstairs?
He wanted to see time fly.
   
Google News Archive
17 March 1956, The Afro-American (Baltimore, MD), “Question Quiz,” Junior Page, pg. 4, col. 3:
Why did the moron throw the clock out of the window?
He wanted to see time fly.
 
10 February 1958, San Diego (CA) Union, “Fun Time—The Riddle Box,” pg. A4, cols. 3-4:
Why did the little boy throw the clock out of the window?
He wanted to see time fly.
 
Google News Archive
8 March 1969, The Deseret News (Salt Lake City, UT), “Fun Time—The Riddle Box,” pg. 2B, col. 2:
Why did the man throw the clock out the window?
To see time fly.
 
Google Books
Children’s Humor:
A Psychological Analysis
By Martha Wolfenstein
Bloomington, IN: Indiana University Press
1978
Pg. 122:
We have already noted several riddles in which the moron wanted to see something (throwing the clock out the window to see time fly; putting the television set on the stove to see Milton Boil).
 
Google Books
Get Thee to a Punnery:
An Anthology of Intentional Assaults Upon the English Language
By Richard Lederer
Layton, UT: Wyrick & Company
2006
Pg. 9:
Why did Silly Billy throw the clock out the window?
He wanted to see time fly.
 
Google Books
The Mammoth Book of Really Silly Jokes:
Humour for the whole family
By Geoff Tibballs
London: Constable & Robinson Ltd
2011
Pg. ?:
Why did the man throw a clock out of the window?
He wanted to see time fly.
 
Google Books
More One Liners, Jokes and Gags
By Grant Tucker
London: The Robson Press
2013
Pg. ?:
Why did a boy throw a clock out the window? To see time fly.