This calculated field returns the number of days an issue was in the "Open" status considering the configured JWT calendar.
Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional.
Number format pattern
Check the parameter Uninitialized when calculated value is zero
Add the field to the relevant view screens.
Remember: All calculated fields are read-only and cannot be manually updated on a create or edit screen.
Check the results
Once configured and added to an issue screen (a parent issue ), the result will look like this: