Here is an automated action that calculated the number of days between twos dates. As the datetime module is available everywhere, it's possible to use it in the automated action. Here is the python expression you can copy/past that return an integer.

(record.x_studio_end_date - record.x_studio_start_date).days 

Result: 

The 'Number of days' is calculated automatically with the Automated Action

The automated action :