Use case
Return the sum of the worklogs of an epic and the issues under it in hours without decimal part.
Expression
Create and configure a Smart field using this expression in General parsing mode:
%{%{issue.issueType} = "Epic" ? (sum(append(fieldValue({issue.timeSpent}, issuesUnderEpic()), [{issue.timeSpent}] )) / 60) : null}
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 to shorten it.