Use case
Return the lowest date from a Date Picker custom field in the linked issues of the current issue.
Expression
Create and configure a Smart field using this expression in Jira expression mode:
%{ count(fieldValue({issue.cfnnnnn}, linkedIssues())) > 0 ? dateToString( min( fieldValue({issue.cfnnnnn}, linkedIssues())), RUN_AS_LOCAL, RUN_AS_LANG) : null}
Please, replace nnnnn with the ID of a Date Picker custom field.