offseting date by integer I've got a content type with fields:
starting date: Date (Unix timestamp)
duration: integer
ending date: Date (Unix timestamp)

User is setting only starting date (year, month, day) and duration (in days)
end after saving the content I would like to automatically calculate ending date (=starting date + duration)

I suppose I should use rules. I tried using actions like set value or calculate value but I failed. Could you help me?
Interval module may help you. It provides an interval field type and also a rules action: "Apply an interval to a date".