110 lines
4.8 KiB
HTML
110 lines
4.8 KiB
HTML
|
<!DOCTYPE html>
|
||
|
|
<html>
|
||
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<!-- This manual documents Guile version 3.0.10.
|
||
|
|
|
||
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
||
|
|
Inc.
|
||
|
|
|
||
|
|
Copyright (C) 2021 Maxime Devos
|
||
|
|
|
||
|
|
Copyright (C) 2024 Tomas Volf
|
||
|
|
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with no
|
||
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
||
|
|
copy of the license is included in the section entitled "GNU Free
|
||
|
|
Documentation License." -->
|
||
|
|
<title>SRFI-19 Introduction (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="SRFI-19 Introduction (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="SRFI-19 Introduction (Guile Reference Manual)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content=".texi2any-real">
|
||
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
||
|
|
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="SRFI_002d19.html" rel="up" title="SRFI-19">
|
||
|
|
<link href="SRFI_002d19-Time.html" rel="next" title="SRFI-19 Time">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
||
|
|
span:hover a.copiable-link {visibility: visible}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<div class="subsubsection-level-extent" id="SRFI_002d19-Introduction">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="SRFI_002d19-Time.html" accesskey="n" rel="next">SRFI-19 Time</a>, Up: <a href="SRFI_002d19.html" accesskey="u" rel="up">SRFI-19 - Time/Date Library</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<h4 class="subsubsection" id="SRFI_002d19-Introduction-1"><span>7.5.16.1 SRFI-19 Introduction<a class="copiable-link" href="#SRFI_002d19-Introduction-1"> ¶</a></span></h4>
|
||
|
|
|
||
|
|
<a class="index-entry-id" id="index-universal-time"></a>
|
||
|
|
<a class="index-entry-id" id="index-atomic-time"></a>
|
||
|
|
<a class="index-entry-id" id="index-UTC"></a>
|
||
|
|
<a class="index-entry-id" id="index-TAI"></a>
|
||
|
|
<p>This module implements time and date representations and calculations,
|
||
|
|
in various time systems, including Coordinated Universal Time (UTC)
|
||
|
|
and International Atomic Time (TAI).
|
||
|
|
</p>
|
||
|
|
<p>For those not familiar with these time systems, TAI is based on a
|
||
|
|
fixed length second derived from oscillations of certain atoms. UTC
|
||
|
|
differs from TAI by an integral number of seconds, which is increased
|
||
|
|
or decreased at announced times to keep UTC aligned to a mean solar
|
||
|
|
day (the orbit and rotation of the earth are not quite constant).
|
||
|
|
</p>
|
||
|
|
<a class="index-entry-id" id="index-leap-second"></a>
|
||
|
|
<p>So far, only increases in the TAI
|
||
|
|
<->
|
||
|
|
UTC difference have been needed. Such an increase is a “leap
|
||
|
|
second”, an extra second of TAI introduced at the end of a UTC day.
|
||
|
|
When working entirely within UTC this is never seen, every day simply
|
||
|
|
has 86400 seconds. But when converting from TAI to a UTC date, an
|
||
|
|
extra 23:59:60 is present, where normally a day would end at 23:59:59.
|
||
|
|
Effectively the UTC second from 23:59:59 to 00:00:00 has taken two TAI
|
||
|
|
seconds.
|
||
|
|
</p>
|
||
|
|
<a class="index-entry-id" id="index-system-clock"></a>
|
||
|
|
<p>In the current implementation, the system clock is assumed to be UTC,
|
||
|
|
and a table of leap seconds in the code converts to TAI. See comments
|
||
|
|
in <samp class="file">srfi-19.scm</samp> for how to update this table.
|
||
|
|
</p>
|
||
|
|
<a class="index-entry-id" id="index-julian-day"></a>
|
||
|
|
<a class="index-entry-id" id="index-modified-julian-day"></a>
|
||
|
|
<p>Also, for those not familiar with the terminology, a <em class="dfn">Julian Day</em>
|
||
|
|
represents a point in time as a real number of days since
|
||
|
|
-4713-11-24T12:00:00Z, i.e. midday UT on 24 November 4714 BC in the
|
||
|
|
proleptic Gregorian calendar (1 January 4713 BC in the proleptic Julian
|
||
|
|
calendar).
|
||
|
|
</p>
|
||
|
|
<p>A <em class="dfn">Modified Julian Day</em> represents a point in time as a real number
|
||
|
|
of days since 1858-11-17T00:00:00Z, i.e. midnight UT on Wednesday 17
|
||
|
|
November AD 1858. That time is julian day 2400000.5.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="SRFI_002d19-Time.html">SRFI-19 Time</a>, Up: <a href="SRFI_002d19.html">SRFI-19 - Time/Date Library</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|