The legislation INT/1922/2003, from the 3rd of July, about registry-books and guest registration forms requires you to fulfil two inexcusable requirements:
Send your guest information to the relevant authorities within the first 24 hours of arrival to the accommodation.
Maintain a registry book in which the information of each guest is collected and their signature. This registry book will be made up of individual sheets called guest registration forms (partes de entrada de viajeros).
As the registration must be done within 24 hours of the guests arrival, registering the guests before they arrive would not be the correct way.
It is also important to bear in mind that the "Organic Law 15/1999, of December 13, on the Protection of Personal Data." prohibits to store, receive or send guest details, for example, their passports or IDs cards, through unsecure methods such as email, Facebook, WhatsApp...etc.
For these reasons, the Check-in Scan app will only allow you to introduce "Today" or "Yesterday" as the Date of Arrival.