Use case
Return the lowest date from a Date Time Picker custom field in the linked issues of the current issue.
Expression
Create and configure a Smart field using this expression in General parsing mode:
%{stringToDate(toString(first(sort(fieldValue(%{issue.cfnnnnn}, linkedIssues()),DESC))), "YYYY-MM-DDTHH:mm:ss")}
Please, replace nnnnn with the ID of a Date Time Picker custom field.
