109 lines
4.8 KiB
HTML
109 lines
4.8 KiB
HTML
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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<title>SRFI-19 Introduction (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="SRFI_002d19.html" rel="up" title="SRFI-19">
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<link href="SRFI_002d19-Time.html" rel="next" title="SRFI-19 Time">
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<div class="subsubsection-level-extent" id="SRFI_002d19-Introduction">
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<p>
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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>
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<hr>
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<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>
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<a class="index-entry-id" id="index-universal-time"></a>
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<a class="index-entry-id" id="index-atomic-time"></a>
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<a class="index-entry-id" id="index-UTC"></a>
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<a class="index-entry-id" id="index-TAI"></a>
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<p>This module implements time and date representations and calculations,
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in various time systems, including Coordinated Universal Time (UTC)
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and International Atomic Time (TAI).
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</p>
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<p>For those not familiar with these time systems, TAI is based on a
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fixed length second derived from oscillations of certain atoms. UTC
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differs from TAI by an integral number of seconds, which is increased
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or decreased at announced times to keep UTC aligned to a mean solar
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day (the orbit and rotation of the earth are not quite constant).
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</p>
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<a class="index-entry-id" id="index-leap-second"></a>
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<p>So far, only increases in the TAI
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<->
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UTC difference have been needed. Such an increase is a “leap
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second”, an extra second of TAI introduced at the end of a UTC day.
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When working entirely within UTC this is never seen, every day simply
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has 86400 seconds. But when converting from TAI to a UTC date, an
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extra 23:59:60 is present, where normally a day would end at 23:59:59.
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Effectively the UTC second from 23:59:59 to 00:00:00 has taken two TAI
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seconds.
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</p>
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<a class="index-entry-id" id="index-system-clock"></a>
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<p>In the current implementation, the system clock is assumed to be UTC,
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and a table of leap seconds in the code converts to TAI. See comments
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in <samp class="file">srfi-19.scm</samp> for how to update this table.
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</p>
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<a class="index-entry-id" id="index-julian-day"></a>
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<a class="index-entry-id" id="index-modified-julian-day"></a>
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<p>Also, for those not familiar with the terminology, a <em class="dfn">Julian Day</em>
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represents a point in time as a real number of days since
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-4713-11-24T12:00:00Z, i.e. midday UT on 24 November 4714 BC in the
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proleptic Gregorian calendar (1 January 4713 BC in the proleptic Julian
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calendar).
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</p>
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<p>A <em class="dfn">Modified Julian Day</em> represents a point in time as a real number
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of days since 1858-11-17T00:00:00Z, i.e. midnight UT on Wednesday 17
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November AD 1858. That time is julian day 2400000.5.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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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>
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</div>
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</body>
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