We will edit the models/library_book.py code file to add a new field and the methods supporting its logic:

1. Start by adding the new field to the Library Books model:

class LibraryBook(models.Model): 
    # ... 
    age_days = fields.Float( 
        string='Days Since Release', 
        compute='_compute_age', 
        inverse='_inverse_age', 
        search='_search_age', 
        store=False, 
        compute_sudo=False, 
    ) 

2. Next, add the method with the value computation logic:

# ... 
from odoo import api  # if not already imported 
from odoo.fields import Date as fDate 
# ... 
class LibraryBook(models.Model): 
    # ... 
    @api.depends('date_release') 
    def _compute_age(self): 
        today = fDate.from_string(fDate.today()) 
        for book in self.filtered('date_release'): 
            delta = (today -
                     fDate.from_string(book.date_release)) 
            book.age_days = delta.days 

3. To add the method implementing the logic to write on the computed field, use the following code:

from datetime import timedelta
# ... 
class LibraryBook(models.Model): 
    # ... 
    def _inverse_age(self):
        today = fDate.from_string(fDate.context_today(self)) 
        for book in self.filtered('date_release'): 
            d = today - timedelta(days=book.age_days)
            book.date_release = fDate.to_string(d)

4. To implement the logic allowing you to search on the computed field, use the following code:

# from datetime import timedelta
class LibraryBook(models.Model): 
    # ... 
    def _search_age(self, operator, value): 
        today = fDate.from_string(fDate.context_today(self)) 
        value_days = timedelta(days=value) 
        value_date = fDate.to_string(today - value_days)
        # convert the operator:
        # book with age > value have a date < value_date
        operator_map = {
            '>': '<', '>=': '<=',
            '<': '>', '<=': '>=',
        }
        new_op = operator_map.get(operator, operator)
        return [('date_release', new_op, value_date)]

An Odoo restart followed by a module upgrade should be needed to correctly activate these new additions.