A joke defines “light-year” much like a “light beer,” in that “it has one-third less calories that an regular year.”
 
“Evan Davis, one of the insane contestants I mentioned, did some first-rate improv with the crowd, stopping to define a light year as having one-third less calories than a regular year” was printed in the San Francisco (CA) Examiner on August 11, 1981. “I (Evan Davis—ed.) once asked my dad what a light year was and he said, ‘It has one-third less calories than a regular year’” was printed in the Sacramento (CA) Bee on September 4, 1985. Evan Davis probably coined (or at least popularized) the joke.
 
 
Wikipedia: Light-year
A light-year, alternatively spelled lightyear, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×1012 km), or 5.88 trillion miles (5.88×1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the time-measurement word “year”, the term light-year is sometimes misinterpreted as a unit of time.
               
Newspapers.com
11 August 1981, San Francisco (CA) Examiner, ‘They laughed when comedy contest started five years ago” by Bill Mandel, pg. A3, col. 2:
Evan Davis, one of the insane contestants I mentioned, did some first-rate improv with the crowd, stopping to define a light year as having one-third less calories than a regular year.
 
Newspapers.com
9 July 1982, Pittsburgh (PA) Post-Gazette, pg. 25, col. 2 ad:
DeAR DIARY
FLASH—I’ve just learned that a lite year has 1/3 less calories than a regular year.
(Froggy’s, a restaurant.—ed.)
 
Newspapers.com
4 September 1985, Sacramento (CA) Bee, “Sinbad finds smooth sailing in comedy competition” by Herb Michelson, pg. AA5, col. 4:
“I (Evan Davis—ed.) once asked my dad what a light year was and he said, ‘It has one-third less calories than a regular year’.”
   
4 September 1985, San Francisco (CA) Chronicle, “Sinbad Sails Through / Surprise winner in comedy marathon” by David Kleinberg, pg. 56:
“A light year, son. That’s one-third less calories than a regular year.”
   
Google Books
Prologue to Action:
Life Sciences Education and Science Literacy
Sponsored by U.S. Public Health Service
Columbus, Ohio
March 1992
Pg. 72:
Will this mean anything to folks who think that a light - year is something that has fewer calories than a regular year and is less filling and tastes great?
 
Google Groups: rec.arts.tv.soaps.misc
DOOL: I learned something new today
DonnaB
Nov 15, 1996, 3:00:00 AM
(...)
~Light year: 1/3 fewer calories than a regular year.
     
Google Groups: alt.sport.bowling
Clear Wolf
William Walker
Jan 11, 1997, 3:00:00 AM
(...)
Q: What’s a light-year?
A: One-third less calories than a regular year.
 
Google Groups: rec.guns
Best caliber/weapon for SHTF
David Steuber
Aug 13, 1999, 3:00:00 AM
(...)
Q: What’s a light-year?
A: One-third less calories than a regular year.
   
Twitter
Br3ndaBot
@Br3ndaBot
Replying to @GuyKawasaki
@guykawasaki: Q: What’s a light-year? A: One-third less calories than a regular year.
1:42 AM · Feb 12, 2008·Twitter Web Client
 
Reddit—Shittyaskscience
Posted by u/supergalactic June 23, 2012
Does a light-year contain less calories than a regular year?
   
Twitter
Ryan Lafleche and 73 others
@RyanLafleche
Fun fact: A light year has 20% fewer calories than a regular year
2:48 PM · Jun 24, 2020·Twitter for Android
 
Twitter
Bob Kostic
@causticbob
What’s a light year?
One third calories less than a regular year.
6:05 PM · Aug 8, 2022·Twitter Web App