Let's start with what we know and work back or forward.
David Townsend married Margaret Hudgins on 28 Aug 1881 in Saline County, Illinois.(Illinois, Marriage Index, 1860-1920 [Ancestry.com.database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2015. Original data: Illinois State Marriage Records. Online index. Illinois State Public Record Offices.)
According to his Death Certificate, son Daniel Lucian Townsend was born on 15 Sept 1886 in Harrisburg (Saline County), Illinois.(Missouri Digital Archives: Missouri Death Certificates 1910-1965; Butler Co. 40742 (http://www.sos.mo.gov/images/archives/deathcerts/1956/1956_00040736.PDF) But, according to his initial Social Security card application, on which is is simply called "Dan Townsend," he was born on 25 Jan 1884 in Saline Co., IL.(U.S., Social Security Applications and Claims Index, 1936-2007 [Ancestry.com.database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2015. Original data: Social Security Applications and Claims, 1936-2007. SSN 251054161)
David and Margaret's son James, according to his World War 1 Draft Registration (Butler Co., Missouri), was born in Illinois on 1 Aug 1894,("United States World War I Draft Registration Cards, 1917-1918," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:K3RH-VZB : 12 December 2014), James Townsend, 1917-1918; citing Butler County, Missouri, United States, NARA microfilm publication M1509 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 1,683,154.) and his application for a Social Security card in 1938 says he was born in Saline Co., Illinois.(U.S., Social Security Applications and Claims Index, 1936-2007 [Ancestry.com. database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2015. Original data: Social Security Applications and Claims, 1936-2007.) His Missouri Death Certificate shows that he was born on 12 Aug 1891.(Missouri Death Certificates: Missouri Digital Archives. Butler #451 (http://www.sos.mo.gov/images/archives/deathcerts/1957/1957_00000451.PDF))
So, David and Margaret were married in 1881 in Saline County, and they stayed they until at least 1 Aug 1894 (or possibly 1 Aug 1891). Both sons were in Missouri for the WW1 Draft Reg., so they moved between 1891/4 and 1918.