Order INT/1922/2003 of 3 July 2003 on registration books and travellers' entry reports requires you to comply with two essential requirements:
Send the details of your guests to the authorities within 24 hours of their arrival at the accommodation.
Maintain a log book in which the details of each guest and their signature are recorded. This log book will consist of individual sheets called: Travellers' entry forms.
As the check-in must be done within 24 hours of the guest's arrival, it is not correct to check-in before arrival.
It is also important to note that the "Ley Orgánica 15/1999, de 13 de diciembre, de Protección de Datos de Carácter Personal." prohibits storing, receiving and sending data of your guests such as their passports and identity cards by unsecured methods such as email, Facebook, WhatsApp, etc.
For these reasons, the Check-in Scan app only allows you to enter "today" or "yesterday" as the check-in date.