This calculated field returns the difference between the current date and the earliest due date among the issue's sub-tasks.
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 parser expression:
Duration display format
For this use case the format duration was chosen since the numeric value is 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.
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: