For this use case the format duration was chosen, since the numeric value being calculated, a number of milliseconds, represents a time duration.
This way the value is represented like 2d 9h 43m instead of a single number.
Check the parameter Use time tracking settings
By ckecking this parameter, the duration will be based the number of working hours per day, and the number of working days per week as configured in time tracking, instead of 24 hours per day and 7 days per week.
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, the result will look like this: