After reading every answer so far, I am going to ofter the most simple of answers.
Every solution here mentions calling date.getTime(). However, this is not needed, as the default conversion from Date to Number is to use the getTime() value. Yep, your type checking will complain. :) And the OP cleary knows they have a Date object, so no need to test for that either.
To test for an invalid date:
isNaN(date)To test for a valid date:
!isNaN(date)