"Up time" is the time period when a system is operational and in service. Up time is the sum of busy time and idle time. For example, if, out of 168 hours, a system has been busy for 50 hours, idle for 110 hours, and down for 8 hours, then the busy time is 50 hours, idle time is 110 hours, and up time is (110 + 50 =) 160 hours.