“To do a great and important work, two things are necessary — a definite plan, and not quite enough time” is a jocular statement about accomplishing goals. “Not quite enough time” means that a sense of urgency is needed to perform a project.
 
The saying has been cited in print since at least 1953 and is of unknown origin. The American composer Leonard Bernstein (1918-1990) has been credited with the saying in many publications in the 2000s, but no early citation credits Bernstein.
     
 
Google Books
28 March 1953, Telephony, pg. 51, col. 2:
It has been said that to do a great and important work two things are necessary — a definite plan, and not quite enough time.
 
Google Books
Teacher’s Treasury of Stories for Every Occasion
By Millard Dale Baughman
Englewood Cliffs, NJ: Prentice-Hall
1958
Pg. 323:
To do a great and important work, two things are necessary—a definite plan, and not quite enough time. — Nuggets.
 
23 January 1963, Laurel (MS) Leader-Call, “Thinking Out Loud: Clever Definitions For Many Words” by Ralph Hays, pg. 4, col. 7:
In order to do an urgent and important work, two things are necessary: a definite plan, and not quite enough time.
 
Google News Archive
2 June 1965, Miami (FL) News, “Pulse of Miami” by Al Volker, pg. 1B, col. 1:
WISH I’D SAID IT FIRST: To do a great and important work, two things are necessary—a definite plan and not quite enough time.
 
Google Books
Quote Unquote
By Lloyd Cory
Wheaton, IL Victor Books
1977
Pg. 373:
Work In order to do an urgent and important work, two things are necessary: a definite plan and not quite enough time. (Grit)
 
27 November 1977, Seattle (WA) Times, “Some sayings say something” by Walt Evans, pg. A10, col. 1:
To do a great an important work, two things are necessary: A definite plan and not quite enough time.
(Collected by Walt Green—ed.)
 
Google Books
How to Run a Successful Design Business:
The New Professional Practice
By Shan Preddy
Burlington, VT: Gower Publishing Company
2011
Pg. 189:
To achieve great things, two things are needed: a plan, and not quite enough time. ~ Leonard Bernstein (1918-1990),
   
Google Books
Insanely Simple:
The Obsession That Drives Apple’s Success
By Ken Segall
New York, NY: Penguin Books
2012
Pg. ?:
Leonard Bernstein captured this thought perfectly when he said:
 
To achieve great things, two things are needed; a plan, and not quite enough time.