“The darkest hour has only sixty minutes”

“The darkest hour has only sixty minutes” is a popular inspirational saying. “Yes, it is true that the darkest hour is only sixty minutes, after all” was cited in 1897. “Don’t forget that the darkest hour is only sixty minutes” was cited in 1898.
 
   
The Contrary Opinion—Buttons
The Darkest Hour Has Only Sixty Minutes
Morris MandelBy Morris Mandel, an American Jewish educator, this button reminds us that troubles pass. As an investor, trouble for you may be a big bear market or a tiny news items that causes your stock to crater. Your job is to look beyond the trouble, remember that good times lie ahead, and take the actions to position yourself to benefit from them.
   
Google Books 
27 October 1897, THe Insurance Press, pg. 4, col. 4:     
POINTERS FOR UNINSURED.
(...)
Yes, it is true that the darkest hour is only sixty minutes, after all.
 
Chronicling America
14 November 1898, Wheeling (WV) Daily Intelligencer, “Don’t” (Chicago Daily News), pg. 4, col. 3:
Don’t forget that the darkest hour is only sixty minutes.
 
6 December 1898, Defiance (OH) Daily Crescent, “Don’t,” pg. 10, col. 3:
Don’t forget that the darkest hour is only sixty minutes.
 
8 November 1911, The Evening News (San Jose, CA), pg. 1, col. 4:
Even in the darkest hour there are only sixty minutes.
 
Chronicling America
10 November 1911, Topeka (KS) State Journal, “Pointed Paragraphs” (From the Chicago News), pg. 4, col. 6:
Even in the darkest hour there are only sixty minutes.
 
24 June 1913, Wilkes-Barre (PA) Times-Leader, “Coal Dust” by A. J. Miner, pg. 8, col. 4:
Cheer Up
Just let your grin stay put, my boy, don’t let your temper sour;
There are only sixty minutes in the darkest hour.
Cincinnati Enquirer
 
15 October 1925, Morning Advocate (Baton Rouge, LA), pg. 9, col. 2:
The darkest hour is only sixty minutes long.
 
15 July 1940, Bellingham (WA) Herald, “Sally’s Sallies” (comic strip), pg. 2, col. 3:
(A child is at the dentist.—ed.)
Even the darkest hour is only sixty minutes.
 
Google Books
1800 Quotes, Quips, and Squibs
By E. C. McKenzie
Grand Rapids, MI: Baker Book House
1972
Pg. 64:
The darkest hour is only 60 minutes long.
 
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“The darkest hour has only sixty minutes.” Morris Mandel
7:01 AM - 15 Dec 2015