### 21.1 Time Basics

Discussing time in a technical manual can be difficult because the word “time” in English refers to lots of different things. In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word “time” for is to talk about the abstract concept.

A calendar time is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC. Sometimes this is called “absolute time”. We don't speak of a “date”, because that is inherent in a calendar time. An interval is a contiguous part of the time continuum between two calendar times, for example the hour between 9:00 and 10:00 on July 4, 1980. An elapsed time is the length of an interval, for example, 35 minutes. People sometimes sloppily use the word “interval” to refer to the elapsed time of some interval. An amount of time is a sum of elapsed times, which need not be of any specific intervals. For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read.

A period is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events. CPU time is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU. CPU time is, therefore, relative to a process. Processor time is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there's a limit to how much can exist in any given interval (that limit is the elapsed time of the interval times the number of CPUs in the processor). People often call this CPU time, but we reserve the latter term in this manual for the definition above.