“Tempus fugit” is a classical Latin saying meaning “time flies. Newspaper columnist Franklin Pierce Adams (1881-1960) wrote in 1916:
 
“Well, well—as the semi-sosh remarked when he watched the elevator indicator move from 1 to 12—doesn’t the time go fast when you’re having fun?”
 
“Time flies when you’re having fun” was printed in The Times-Picayune (New Orleans, LA), in 1933. The Man Who Came to Dinner,  a comedy in three acts by George S. Kaufman and Moss Hart that opened at the Music Box Theatre in New York City on October 16, 1939, popularized the line to a large audience.
 
“Time flies when you’re having rum” and “Wine flies when you’re having fun” are related sayings. “Time’s fun when you’re having flies, Kermit said” and “What did one frog say to the other?”/“Time’s sure fun when you’re having flies” are jokes on the saying that transpose words.
 
 
Wikipedia: Tempus fugit
Tempus fugit is a Latin phrase, usually translated into English as “time flies”. The expression comes from line 284 of book 3 of Virgil’s Georgics, where it appears as fugit inreparabile tempus: “it escapes, irretrievable time”. The phrase is used in both its Latin and English forms as a proverb that “time’s a-wasting”. Tempus fugit, however, is typically employed as an admonition against sloth and procrastination (cf. carpe diem) rather than a motto in favor of licentiousness (cf. “gather ye rosebuds while ye may”); the English form is often merely descriptive: “time flies like the wind”, “time flies when you’re having fun”.
 
Wiktionary: time flies when you’re having fun
Proverb
time flies when you’re having fun
1. Time seems to pass more quickly when one is enjoying oneself
 
12 August 1916, Pittsburgh (PA) Post, “The Conning Tower” by F.P.A., pg. 6, col. 6:
Well, well—as the semi-sosh remarked when he watched the elevator indicator move from 1 to 12—doesn’t the time go fast when you’re having fun?
 
Chronicling America
28 May 1919, New York (NY) Tribune, “The Conning Tower” by F.P.A., pg. 12, col. 4:
The unalcoholic days, the saddest of the year, are not yet to come, and house stories continue to be told, most of them dull. Our favorite drunk yarn is Ted Robinson’s, about the gentleman who, gazing at the elevator indicator as the car went from the main floor to the 12th, said, “Say, don’t the time go fast when you’re having fun?”
 
29 October 1933, The Times-Picayune and New Orleans States (New Orleans, LA), “Prize Letter,” The Young People’s Paper, pg. 3, col. 2:
When everyone finally got an apple it was almost 11 o’clock. (Time flies when you’re having fun.)
(...)
BARBARA AITKENS
Houma, La.
 
Google Books
Best Plays of the Modern American Theatre
Edited by John Gassner
New York, NY: Crown Publishers
1939
Pg. 360:
WHITESIDE. He is, my dear. Just the family circle gathering at Christmas. (A look at his watch.) My, how time flies when you’re having fun.
 
OCLC WorldCat record
Rad Ed, or, How Time Flies When You’re Having Fun
Author: Henry Fernandez
Edition/Format: Article Article : English
Publication: College English, v35 n3 (19731201): 317-320
Database: JSTOR Language & Literature Collection
 
OCLC WorldCat record
Time flies when you’re having fun, and when you’re not
Author: Al Cadenhead
Publisher: Nashville, Tenn. : Broadman Press, ©1991.
Edition/Format:   Print book : English