[Mauzen]

Its actually just using the remaining seconds, and calculates the days from it. If the expiration time is in less than 24 hours, it will show "0 days remaining". So if it e.g. expires at 1AM the next day, and currently its 11PM it will also show 0 days. Is this the behaviour you want? Or should it actually consider the date, so in the "expires-at-1AM" example it would show 1 day until its 0AM?
This would be more work then, and its hard to do that accurately as players timezones dont have to be the same as the servers timezone.