The date is still a FileMaker date field that you are Selecting. If your calculation field specifies a date result type, you should get either a date, null or a ? result. Null means no record matching the specified criteria was found and a ? of course means that you have a syntax error in your query. In cases where you are not using this in a calculation field, you might need to enclose the results in a GetAsDate function to make sure that the value is not returned as text.
The place where dates cause problems in ExecuteSQL are when you need to specify a literal date in the expression. Then you have to follow the rules for using a SQL compatible date format.