A formula is going to have a type based on the result of the formula.
1=1 boolean
Date(somestring) date or datetime
What DB type is the date in SQL server Express Varchar or datetime? If the type is a datetime, I think it would be much easier to use the field format options to control how the date is displayed, and that is inside of Crystal, not part of the VBCode.
I would send the report a parameter and not a formula. Place the field on the report twice. Find the supress option and supress the field based on the value of the parameter. Only one will show at a time, based on what your user checks in the VB.net application.
I guess you could use a formula that has a result of type Boolean. Place that formula in the fx section next to the supress checkbox.