Left expression is not same type as right expression

Left expression is not same type as right expression

I've been trying to figure this out and still have not gotten an answer back from support on this so I thought I'd see if anyone here knew the answer.  I am trying to populate a date lookup field. I have a registration form with a list of classes (from form Class_List) which has a lookup for StartDates.  On my registration form I use a lookup to get the class list (narrowing it down within the program) and then want the next dropdown to only show the start dates for that class based on the class name (from the Class_LIst form).  I have attached a screenshot of the lookup field I'm working on.  The field in the Class_List is "StartDate" and the field on the Registration Form is "Start".  Here is the error I keep getting:

Error in statement at line number 0
The Left expression is of type TIMESTAMPE and the right expression is of type STRING and the operator == is not valid here.  

How do I convert this so they are the same type (or do I need to use a different operator to limit results to just that class).

Thanks in advance.