Use case
Elapsed time between creation and resolution in days.
Expression
Create and configure a Smart field using this expression in General parsing mode:
%{ {issue.resolutionDate} != null ? (({issue.resolutionDate} - {issue.created}) / DAY ) : null }
The duration can be retuned in years, weeks, days, hours or minutes if the DAY keyword is replaced in the above expression with the corresponding time constant.