The first reference to his birth that I have located is the 1900 Census. The census lists his age at 8 years of age born in August of 1891. He had two siblings listed that were older than him, one born in 1881, and the other in 1889. One child was listed after Otis;she was born in October of 1893.
The 1910 census record states the answers noted in the census about person was as of the date of April 15, 1910. Otis was listed as age 18 which would be his age at his last birthday in August of 1891.
The next reference to his age was located in in the World War I Draft Registration Card. His date of birth is listed as August 20, 1891 and he states that he was born in St. Elmo, Illinois.
The 1920 Population Census lists Otis living in Dora, Moultrie, Illinois. This record indicates that he is 27 years of age as of the 3rd of January 1920. This information indicates a birth year of 1892. At this point Otis was living with his first wife’s family and more than likely one of the Butts family gave the information to the census taker.
The 1930 Population Census finds the Otis married to Ruby [Poundstone] Clark living in Unity Township, Piatt County, Illinois. His age is listed as 38, which would make his birthdate in 1891. The census date was April 9th of 1930 so his age would refer to his last birthday in August of 1929.
The 1940 census record indicates a birth year of 1892. The 1940 Population Census taking in April of 1940 lists and age of 47 which would correlate with an 1892 birth year.  Otis’ World War II Registration Card, dated 27 April 1942 lists a birth date of August 20, 1892. References to his age after the 1940 census including his death certificate,memorial card,obituary,delayed birth certificateand Ruby Clark’s typewritten listof birth dates all agree with the August 20, 1892 birth date.
My conclusion is that Otis was born on August 20, 1891. I believe the earlier records are correct and the later records are incorrect. It appears that Grandpa made himself a year younger possibly by accident or perhaps by a failing memory.
|WWI see footnote 3|
|Newspaper Clipping, obituary of Otis Clark, see footnote 9|
 1900 U.S. Census, Fayette County, Illinois,population schedule, Louden Township, enumeration district (ED)19,Page 3B,Household 57, Charles and Mary Clark family; digital images, Ancestry.com(www.ancestry.com : accessed May 1 2011; United States of America, Bureau of the Census. Twelfth Census of the United States, 1900. Washington, D.C.:National Archives and Records Administration, 1900. T623, roll 300.
 1910 U.S. Census, Fayette County, Illinois,population schedule, Loudon Township, enumeration district (ED) 63, 3A(stamped), dwelling 42, family 42, Henryand Bessie Clark household; digital images, Ancestry.com(www.ancestry.com : accessed 9 June 2009); Thirteenth Census of the UnitedStates, 1910 (NARA microfilm publication T624, film 1374300.
 "World War I Draft Registration Cards,1917-1918," digital image, Ancestry.com (www.ancestry.com :accessed 3 July 2011), entry for Otis E. Clark, Registration Card 48, MaconCounty, born 20 Aug 1891, stamped 12-6-13-A; citing World War I SelectiveService System Draft Registration Cards, 1917-1918., Family History Roll#1614322.
 1930 U. S. Census, Piatt County, population schedule, UnityTownship, enumeration district (ED) 15, page 3, dwelling 69, household 71,Family of Otis and Ruby Clark; digital images, Ancestry.com(www.ancestry.com : accessed 11 August 2012); United States of America, Bureauof the Census. Fifteenth Census of the United States, 1930. Washington, D.C.:National Archives and Records Administration, 1930. T626.
 1940 U. S. Census, Christian County, population schedule,Bear Creek, enumeration district (ED) 11-4, 3B (stamped), household 52, Familyof Otis and Ruby Clark; digital images, Ancestry.com: accessed 10 August2012); Original data: United States of America, Bureau of the Census. SixteenthCensus of the United States, 1940. Washington, D.C.: National Archives andRecords Administration, 1940. T627, 4,643 rolls.
 "U.S. World War II Draft Registration Cards,1942," digital image, Ancestry, Ancestry.com Operations(www.ancestry.com : accessed 16 February 2012), entry for Otis Elmer Clark,serial number U1367; citing U.S. World War II Draft Registration Cards, 1942,National Archives and Records Administration (NARA); Washington, D.C.; StateHeadquarters: Illinois; Microfilm Series: M2097; Microfilm Roll: 49.
 Otis E. Clark, death certificate Registration Dist. # 11,Registered #76-190 (1976), Christian County Clerk's Office, Taylorville,Christian County, Illinois.
 Memorial Card for Otis E. Clark, Connolly Funeral Home;Clark Family Artifacts; privately held by Amy Urman, [address for private use], Tucson, AZ 85711, 2011. MemorialCard from the funeral of Otis E. Clark. Amy Urman received the card at thefuneral of Otis E. Clark on November 17, 1976.
 Newspaper clipping, obituary of Otis E. Clark, Clark Family Artifacts; privately held by Amy Urman, [address for private use], Tucson, AZ 85711, 2011. Otis E. Clark, 84, Stonington Dies Suddenly Today in Home. original clipped by Ruby Clark, removed from her home by Kent Clark after her death, photo taken of obituary at Kent Clark's residence in 2007.
 Otis Elmer Clark, birthcertificate Registration #6483 (12 August 1892), Town Clerk, Beecher City,Effingham County, Illinois.
 Typewritten list of family birth's and death's found among Ruby Clark's papers, Clark Family Artifacts; privately held by Amy Urman, [address for private use], Tucson, AZ 85711, 2011. Original document is in possession of Kent Clark, photographed by Amy Urman June 2007.