Although this sounds like a strange question, why are you trying to check something to see if it's a date? If you intend to CONVERT it without having a failure, use TRY_CONVERT() or TRY_CAST() instead. If it's a valid DATE or DATETIME, it'll return it in the DATATYPE you specified. If it's not valid, it'll return a null.
Also, when you have a question on a function of this nature, it'll take you less time to look it up than to ask the question on a forum. See the following and look for the RETURN TYPE (an INT, in this case) and a couple of the examples.