All you really need to do is subtract the current date from the
warranty expiration date. If the result is less than zero, the
warranty has expired.

If you want to show the number of days past expiration, you could use
something like this, using B8 for an example:

=IF(B8<TODAY(),ABS(B8-TODAY()),"")

Format the cell as General.

This shows blank if the warranty has not expired, and the number of
days past warranty once expired.