Dark Thirty (Zero Dark Thirty; Oh Dark Thirty; 0 Dark 30; 0 Dark 100)

“Dark thirty” (or “dark-thirty”) means thirty minutes after the sun goes down. The earliest citations for “dark thirty” appear to be from Texas and New Mexico in the 1940s-1950s.
 
“Dark thirty” crept into military jargon, with “0 dark 30” (also “zero dark thirty,” “oh dark thirty,” “o’dark thirty”) being used by the 1970s. It is often claimed that “dark thirty” began in the military, but no citations have been found before the 1960s-1970s (Vietnam War era). The nonsense “0 dark 100” has also been used.
   
It should be noted that the military use of “0 dark 30” refers to thirty minutes before the sun rises (very early in the morning), not thirty minutes after the sun sets (Texas usage).
 
   
Answers.com
0 Dark 30
Marine Corps Dictionary: 0 Dark 30
Very early in the morning, pronounced oh, dark thirty. [Editor’s Note: A lot of people have complained that a Marine would not call the numeral zero an “oh”. That is possibly correct but it was pronounced in that unorthodox way when it was first used in the 1960s, possibly to add emphasis to the phrase.]
 
SlangSite.com
O-Dark Thirty: a ridiculously early hour in the morning
Example: We will meet in front of the hotel at O-dark thirty…and then we will take the subway to the race start
O-dark-hundred-hours: Really early in military slang. Modeled after terms like 0700 hours.
Example: Missed my flight and flew standby, to arrive at o-dark-hundred-hours. 
 
Urban Dictionary
dark-thirty
the thirty minutes before nightfall, or the thirty minutes after night falls.basically the 1 hour time frame as it becomes dark.
i told david the party starts at dark-thirty.
by da trick biatch Dec 20, 2005
   
dark-thirty
It is actually o’ dark-thirty. It is a military term refering to beinging somewhere before the sun rises
We have to go, we have PT (physical training)at 0’ dark-thirty.
by Goldi Dec 21, 2005
 
Urban Dictionary
darkthirty
Something hicks from illinois say when they’re referring to the time right after the sun sets.
Be home before darkthirty ya’ll.
by AlissaGrace Apr 1, 2005
 
Google Books
The New Mexico Quarterly
By University of New Mexico
Published by University of New Mexico
Item notes: no.17
1947
Pg. 196:
And by dark thirty vacas have been used up, and many young men have been carried out senseless, and when the band tunt-tunts the end of the last vaca, a great Spanish shout goes up and everyone is suddenly courageous and two hundred rush at the beast in the dusk…
 
10 December 1950, Big Spring (TX) Daily Herald, pg. 12, col. 1 ad:
To wear at “Dark Thirty” and after…exciting evening dresses in both short and long lengths. From $35.00.
(Swartz’s—ed.)
 
11 December 1955, Abilene (TX) Reporter-News, “The Waggin’ Tongue” by Bob Cooke and Duane Howe, pg. 13D, col. 5:
Wednesday evening, about dark thirty, a truck driver arrived in Abilene from Beaumont with 70-odd Brahman calves for delivery to the Abilene Range Riders for use in the Jan. 1 matched and jackpot roping events being staged by the Range Riders.
 
30 September 1958, Waco (TX) News-Tribune, “Freels Catches Bass Hitting Over 6 Pounds” by Earl Golding, pg. 8, col. 4:
Freels and his partner, Twelfth AP Sergeant Jim LaRue, put into the water from Elmer Hill’s around dark thirty.
   
14 May 1959, Middlesboro (KY) Daily News, pg. 6, col. 4:     
“WHAT TIME IS IT!”
I have lost track of time, but I think it’s about dark thirty.
 
3 June 1962, Las Cruces (NM) Sun-News, pg. 14, col. 7 classified ad:
Highland Manor—Open House
3 P.M. ‘Til Dark Thirty
 
25 July 1963, Abilene (TX) Reporter-News, “‘Hey Day’ Doings Set At Haskell,” pg. B1, col. 1:
...and a dance billed as beginning at “dark thirty” for all those able to dance.
 
8 September 1969, Abilene (TX) Reporter-News, “Gates Open, And 1969 Fair Here,” pg. 1A, col. 2:
Kids and grown-ups alike have been “Fair” talking for weeks and before dark-thirty the Fair will be in full swing. 
   
25 February 1970, Lubbock (TX) Avalanche-Journal, pg. C11, col. 2 classified ad:
Our three and four bedroom homes range in price from $17,600 to $21,650 and are open 7 days a week until dark thirty.
 
10 April 1970, Port Arthur (TX) News, “Crawfish Festival to Open” by Ruth Crane, pg. 7, col. 7:
A variety of dancing is on the program including a Cajun “Fais-Do-Do,” square dancing, a Cajun snake dance, a citizens band radio club coffee break and dance in the park at 6:30 p.m. and a Rodair club Cajun dance on Highway 365 beginning at “dark-thirty.”
 
12 June 1975, Kerrville (TX) Mountain Sun, “Tolbert’s Texas” by Frank X. Tolbert (from the Dallas Morning News), pg. 8, col. 4:
The announcement reads that the “Crazy Comfort Little Theater” at nearby Grapeton, will have a drama on the nights of May 9-10, the performances to start at “at dark-thirty.”
 
Janie Stratton translated: “At dark-thirty means the play in the Crazy Comfort Little Theater will start 30 minutes after the sun goes down.”
 
3 February 1977, Dallas (TX) Morning News, “Tolbert’s Texas” by Frank X. Tolbert, section D., pg. 3, col. 1:
The English language is sometimes arranged uniquely in the Hill Country. For sample, if someone wants to tell you not to come to a party until after dark they will write it “thirty dark,” meaning 30 minutes after the sun goes down.
 
6 December 1981, Washington (DC) Post, “The Sked-Busted, Slam-Clicking Stewardess” by Nancy Ballard, pg. C3, col. 2:
A typical sentence might be, “I went illegal for 228 to LAX and crew sked busted my sequence, but that’s okay with me ‘cause I’ll lay over, sign in at zero dark hundred, deadhead to base and still get my 4-1/2.”
Translation:  “I can’t work my scheduled flight because I’ve been on duty too long, so I’ll stay overnight here, get up early and ride as a passenger to my home base.  I’ll still get paid.”
     
Google Books
A Forgiving Wind: On Becoming a Sailor
By Fred Powledge
Published by Sierra Club Books
1983
Pg. 174:
Andy lent us his dinghy, which we towed behind Arena, and we got going early, at what he referred to as “Oh-dark-thirty in the morning.”
   
Google Books
A Dictionary of Soldier Talk
By John Robert Elting, Dan Cragg, Ernest L. Deal
New York, NY: Scribner
1984
Pg. 214:
o-dark-thirty (Modern)
     
OCLC WorldCat record
Dark thirty : a novel
by Terry Kay
Type:  Book; English
Publisher: New York : Poseidon Press, ©1984.
 
23 December 1984, New York (NY) Times, Jill Grossman book review of Terry Kay’s 1984 novel “Dark Thirty,” pg. BR17:
Jesse Wade, the patriarch of a rural Georgia family, returns home one evening at “dark thirty” (the half-hour between day and night) to find his entire family brutally murdered.
 
3 December 1990, Business Week, “No Answering Machine Ever Looked Like This” by Joan O’C. Hamilton, pg. 133:
In sessions that dragged late into what frog employees call ‘‘0-dark-hundred hours,’’ Harden and Braund noodled, doodled, and sketched the possibilities.
 
16 December 1991, Portland (OR) Oregonian, “Duck Hunting Means More Than Capturing the Game” by Bill Monroe, pg. C3:
It is Oh-dark-thirty (as military veterans sometimes recall their predawn reveilles) and the boat is slicing across the glass-smooth surface of Scappoose Bay, up the glimmering orange path of a distant mill light.
 
10 November 1991, Syracuse (NY) Herald Journal, “The Hunt Is On, But Where’s The Cat?” by Marie Villari, section 1, pg. 1, col. 1:
It is way before dawn when the alarm clock goes off. O’dark thirty, he calls it. I call it an ungodly hour.
 
OCLC WorldCat record
The dark-thirty : Southern tales of the supernatural
by Pat McKissack;  J Brian Pinkney
Type:  Book : Fiction : Juvenile audience; English
Publisher: New York : Knopf, ©1992.
   
OCLC WorldCat record
Dark30.
Type:  Journal, magazine : Periodical; English
Publisher: Houston, TX : Susan R. Higgins : Hilton Jackson, ©1993
 
OCLC WorldCat record
O’dark thirty, Vietnam
by U.S. Fighter Squadrons, Inc.;  Atwood Keeney and Company.
Type:  VHS video; English
Publisher: Sunrise, FL : U.S. Fighter Squadrons, Inc., ©1995.
 
OCLC WorldCat record
It’s Oh-Dark Thirty. Do You Know Where Your Enemy Is? The Importance of Datums When Locating Your Enemy
by James M Giesken;  AIR COMMAND AND STAFF COLL MAXWELL AFB AL.;
Type:  eBook; English
Publisher: Ft. Belvoir Defense Technical Information Center MAR 1997.
 
OCLC WorldCatrecord
Zero dark thirty
by Samuel Brantley
Type:  Book; English
Publisher: Central Point, Or. Hellgate Press 2002
 
OCLC WorldCat record
Commentary - Flight Log - Truly Oh-Dark-Thirty
by Jim Humphreys
Type:  Article; English
Publisher: New York : Reed Publications, 1966-
Publication: Business and commercial aviation. 98, no. 5, (2006): 108
Database: ArticleFirst
   
OCLC WorldCat record
Zero-dark-thirty
by Alexis Burgdorf Sullivan
Type:  Thesis/dissertation; English
Publisher: 2008.
Dissertation: Thesis (M.F.A.)—Sarah Lawrence College, 2008.