in does not support text and date measures Smart View does. The number you are seeing is the number of seconds since Jan1.1970 which is how Essbase internally stores dates. I also don't think you will be able to get the value o the UDA to use in a calculation. you might need to create another measure Monthend date that you populate with the date you want to calculate from.
Retrieve it in Smart View and see if it is working? Sure you uploaded 7/31/1996 that number is the equivalent for 7/30/1996.
Essbase is setting the right value (the number you see there is called as Epoch time). It should be working fine. Did you try just the @DATEPART first?
@DatePart("Store Open Date",DP_MONTH);
EDIT: You beat me (Smart View) GlennS_3