[otrs] work units and effective time of resolution

Hi, are work units expressed in hours or minutes or is it only a generic number field? I'm trying to find a method to calculate the effective time of resolution of a ticket. I have enable my working hours in the admin console but the differences between change_time and create_time doesn't consider it. Are WorkingHours, TimeVacationDays useful only in the Escalation circumstances? I don't know how to do with external tools L Thank you, Ale