[otrs] Due Date calculation

Hello, I have created a queue SEVERITY-3 with Calendar 1(8:00-17:00 x 5). Calendar 1 is configured as 8:00-17:00 for weekdays. Escalation solution time for that queue is 14400 mins (10 days). While creating an e-mail ticket, Due Date is given as TOMORROW, my escalaiton solution time constraint is not taken into consideration. How can I make DUE DATE calculated to 10 days later. Thanks in advance, Aysel