Javascript add days to date

I created a trigger that updates a date (DueDate) field and adds 28 days to a date that is another date field (IssuedDate) in the object.

The code is the following:

var issueDate = new Date({!Issued_Date#iso});
var dueDate = new Date();
dueDate.setDate(issueDate.getDate() + 28);
return dueDate;


But it doesn't work as expected.

Thank you in advance.

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