Note about the solution, the difficulty was caused by hidden characters in the variable "Mark_App_s ", which was imported from an online database.  Prior to implementing the above solution, I had to parse the variable using a SUBSTRING into MM, DD, YYYY components, Then tried converting each to a numeric variable.  If it would not convert using an INPUT function, that indicated there were hidden characters, and I continued to use the SUBSTRING function until the numeric conversion method worked.  Only then could the 

Mark_App_d  =     mdy(Month_2,Day,Year)       

function be used to convert the character variables (Month_2,Day,Year) to a SAS Date.

Thanks again for your assistance !