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<h3 class="section">21.1 Time Basics</h3>

<p><a name="index-time-2588"></a>
Discussing time in a technical manual can be difficult because the word
&ldquo;time&rdquo; 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 &ldquo;time&rdquo; for is to talk about the abstract concept.

   <p>A <dfn>calendar time</dfn> is a point in the time continuum, for example
November 4, 1990 at 18:02.5 UTC.  Sometimes this is called &ldquo;absolute
time&rdquo;. 
<a name="index-calendar-time-2589"></a>
We don't speak of a &ldquo;date&rdquo;, because that is inherent in a calendar
time. 
<a name="index-date-2590"></a>
An <dfn>interval</dfn> 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. 
<a name="index-interval-2591"></a>
An <dfn>elapsed time</dfn> is the length of an interval, for example, 35
minutes.  People sometimes sloppily use the word &ldquo;interval&rdquo; to refer
to the elapsed time of some interval. 
<a name="index-elapsed-time-2592"></a><a name="index-time_002c-elapsed-2593"></a>
An <dfn>amount of time</dfn> 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.

   <p>A <dfn>period</dfn> is the elapsed time of an interval between two events,
especially when they are part of a sequence of regularly repeating
events. 
<a name="index-period-of-time-2594"></a>
<dfn>CPU time</dfn> 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. 
<a name="index-CPU-time-2595"></a>
<dfn>Processor time</dfn> 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. 
<a name="index-processor-time-2596"></a>

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