Can't cast a SPROC parameter?

I have several sprocs that take a date/time input, one of which takes a date only in the form of an nvarchar because it utilizes dynamic SQL. When I try to cast the variable going into the sproc I'm met with an error. This doesn't make much sense to me.

I know I can work around it by creating a second set of nvarchar variables or casting within the sproc, but I'd like some insight on why I can't just cast the variable in-line.

Example. TOP is a Select statement telling me my NVARCHAR DATE kicks out just fine. MID is where SSMS is throwing the error, BOTTOM is where SSMS thinks it's just fine.