Hi,

In Odoo Online you can make use of the Odoo Studio app to do this. To do that please follow the below steps:-

1) Go to settings and turn on the developer mode by clicking the "Activate the developer mode" button under the Developer Tools
2) Now go to the view in which you need to add the field.
3) Now switch to studio mode by clicking on the "Toggle Studio" button found in the top right corner.
4) Now add and name one date field and an integer field using the studio. (You can refer to Odoo Studio Book for help: https://www.cybrosys.com/odoo/odoo-books/odoo-15-studio/build-an-app/creating-models-and-adding-fields/)
5) Now we need to add compute method for the integer field for that you can refer to Odoo Studio Book: https://www.cybrosys.com/odoo/odoo-books/odoo-15-studio/build-an-app/define-compute-method/
6) In "Dependencies" you need to add the technical name of the date field which you will get from studio view or just by placing the name over the date field(which will be similar to "x_studio_input_date").
7) Now in the "Compute" field you need to enter the following code "for rec in self:
rec['x_studio_date_of_the_year'] = rec.x_studio_input_date.strftime('%j')"
8) Now you can save it and close the studio view.

Now the integer field will show the day of the year when the value in the input date field is changed

Regards