How to setup 24x7 SLA for tickets

If the customer has a 24x7 service time with multiple global regions, how do we setup the SLA - working day calendar and operating hours. When you create a service level in Hybris Service, we have to set the working day calendar, time zone and operating hours per day in a week. As per above requirement I selected General calendar for 7 days a week, no time zone and no operating hours. As per help document if you do not set operating hours then system assumes 24x7. However when trying to activate I get error to select time zone and operating hours. How do we setup for 24x7 without errors.