This calculated field returns the number of hours that an issue has had the current priority. The value of the field will change when the Priority field is updated with a different value.
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.
Add the following expression:
Number format pattern
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.