"Expires at" field at users screen not working right

Hi all.

After activating "Pay per View", a field "Expires at" appears for each user at users administration.
No matter what date you fill it with, past or future, because if you don't leave this field empty, it allows that user seeing the content.
I think the correct behaviour would be not allowing to access the content if "Expires at" is filled with a date in the past. And allowing access if that date is a future date.

Could you solve this, please? Thanks in advance