The Town of Delmar was founded in October 1859 with the extension of the Delaware Railroad to the southern boundary of Delaware. The Transpeninsular Line was responsible for the founding of this unique bi-state town because the Charter of the Delaware Railroad Company permitted only the building of a railroad within the State of Delaware and the charter of the corresponding railroad company in Maryland permitted only the laying of railroad track within the State of Maryland. Thus, in 1859, the two respective railroads met and the Town of Delmar was born. The name of Delmar was derived for this railroad center from the states whose line it straddles - DELaware and MARyland.
The Delaware Public Archives says it was December 20th 1859 that Delmar was founded so lets just say in the last quarter of 1859 Delmar Became Delmar.