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.
If the resulting number from the expression contains too many decimal numbers, feel free to select one of the available display formats or to configure it manually.