*I'm quite the fan of definitions of time concepts. In this case it's about mil-spec electronics.
Definitions of Time Concept
CHAPTER 14: MIL-HDBK-338, VOLUME I
Time is of fundamental importance in the qualification of the basic terms defined above. In general, the interval of interest is the total calendar time in which an item or system is in use. This interval may be divided into required time and non-required time.
Active time is that during which an item is in an operational inventory; inactive time is that during which an item is in reserve. Active time may be further broken down into up-time (during which an item is in a condition to perform a required function) and downtime (during which an item is not in a condition to perform a required function). Downtime may be further subdivided into maintenance time (that downtime which excludes modification and delay time), modification time (that downtime necessary to introduce any specific change(s) to an item to improve its characteristics, or to add new ones), and delay time (that downtime during which no maintenance is being accomplished on the item because of either supply or administrative delay). Delay time may be further subdivided into supply delay time (that element of delay time during which a needed replacement item is being obtained) and administrative time (that element of delay time not included in supply delay time).
Maintenance time can be broken down into corrective maintenance time (during which corrective maintenance is performed on an item), and preventive maintenance time (during which preventive maintenance is performed on an item).
Uptime can be further subdivided into: not operating time (during which the item is not required to operate), alert time (during which an item is assumed to be in specified operating condition, and is awaiting a command to perform its intended mission), reaction time (that element of uptime needed to initiate a mission, measured from the time command is received), and mission time (during which an item is required to perform a stated mission profile).
