[otrs] effective time of ticket resolution

Hi, I've a question for my Otrs My WorkinHours begins form 9.00 o'clock to 18.00. I open a ticket at 17.00 and I close it next day at 12.00. How can I know the effective time of this ticket? (That It would be 4 hours) In the history of ticket I see only the age of time which is increasing continuously. Thank you! Alessandro