📄 leapyear.html
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:47 2005 by faqproc version 2.7 --><!-- from source file algorithms.sgml dated Sat Feb 14 14:35:07 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/leapyear.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.32</title><link href="zeller.html" rev=precedes><link href="leapsec.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="zeller.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="leapsec.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 20.32</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Is <TT>(year % 4 == 0)</TT> an accurate test for leap years?(Was 2000 a leap year?)</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>No, it's not accurate(and yes, 2000 was a leap year).Theactual rulesfor thepresentGregorian calendarare that leap years occur every four years,but not every 100 years,<em>except</em>that they do occur every 400 years, after all.In C,these rules can be expressed as:<pre> year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)</pre>See a goodastronomical almanac or other reference<a href="refs.html" rel=subdocument>[footnote]</a>for details.</p><p>Actually,if thedomain of interest is limited(perhaps by therange of a <TT>time_t</TT>)such thatthe only century year it encompasses is 2000,the expression<pre><TT>(year % 4 == 0) /* 1901-2099 only */</TT></pre>is accurate,if less than robust.</p><p>If you trust the implementor of the C library,you can use <TT>mktime</TT> to determine whether a given year is a leap year;see the code fragmentsin questions <a href="../lib/calendar.html">13.14</a> or <a href="zeller.html">20.31</a>for hints.</p><p>Note also that the transition from theJulian to theGregorian calendar involveddeleting several daysto make up for accumulated errors.(The transition was first madein Catholic countries under Pope Gregory XIIIin October, 1582,and involved deleting 10 days.In the British Empire,elevendays were deletedwhen the Gregorian calendar was adoptedin September 1752.A few countries didn't switch until the20thcentury.)Calendar code which has to work forhistorical datesmustthereforebe especially careful.</p><p>See also question <a href="../lib/y2k.html">13.14b</a>.</p><!-- aend --><p><hr><a href="zeller.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="leapsec.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/misc/leapyear.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --></html>⌨️ 快捷键说明
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