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请输入四位年份：<input type="text" id="mytext"/><input type="button" value="检验" onclick="isLeapYear()">
<script>
function isLeapYear(){
var year = document.getElementById("mytext").value
if(year%400==0 || (year%100!=0 &&year%4==0)){
alert(year+"年是闰年!")
}else{
alert(year+"年是平年!")
}
}
</script>
</BODY>
</HTML>
知道了原理其实很容易，要判断是否是闰年，有两种方式：
1，闰年的计算规则：
普通闰年：公历年份是4的倍数，且不是100的倍数的，为闰年（如2004年、2020年等就是闰年）。
世纪闰年：公历年份是整百数的，必须是400的倍数才是闰年（如1900年不是闰年，2000年是闰年）。
所以只要判断年份是否是400的倍数，或者是4的倍数且不是100的倍数即可，这个楼上已经给出答案了。
这种判断方式是所有语言通用的，适用于1852年之后的所有闰年判断。
2，只需要查看二月的天数是28天还是29天即可，利用构造函数 Date() 的传参特性可以轻松实现。
以下代码为 JavaScript 代码，其它语言原理相通，代码需要自己根据原理自行修改。
var y = 2020//要判断的年份
var d = new Date(y, 1, 29)// 参数的年份由y决定，后面两个则是固定值
var isLeap = d.getMonth() == 1// true 为闰年，false 为平年
console.log( y + "年为" + (isLeap ? "闰年": "平年" ) + "， 2月有" + (isLeap ? 29: 28) +"天" )
代码提示：将下面的代码复制到〈BODY〉区<script language="JavaScript">
<!--
function CalConv()
{
FIRSTYEAR = 1998
LASTYEAR = 2031
today = new Date()
SolarYear = today.getFullYear()
SolarMonth = today.getMonth() + 1
SolarDate = today.getDate()
Weekday = today.getDay()
LunarCal = [
new tagLunarCal( 27, 5, 3, 43, 1, 0, 0, 1, 0, 0, 1, 1, 0, 1, 1, 0, 1 ),
new tagLunarCal( 46, 0, 4, 48, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1 ), /* 88 */
new tagLunarCal( 35, 0, 5, 53, 1, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1 ), /* 89 */
new tagLunarCal( 23, 4, 0, 59, 1, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1 ),
new tagLunarCal( 42, 0, 1, 4, 1, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1 ),
new tagLunarCal( 31, 0, 2, 9, 1, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0 ),
new tagLunarCal( 21, 2, 3, 14, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 1 ), /* 93 */
new tagLunarCal( 39, 0, 5, 20, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1 ),
new tagLunarCal( 28, 7, 6, 25, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 1 ),
new tagLunarCal( 48, 0, 0, 30, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 1 ),
new tagLunarCal( 37, 0, 1, 35, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1 ), /* 97 */
new tagLunarCal( 25, 5, 3, 41, 1, 1, 0, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1 ),
new tagLunarCal( 44, 0, 4, 46, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1 ),
new tagLunarCal( 33, 0, 5, 51, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1 ),
new tagLunarCal( 22, 4, 6, 56, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0 ), /* 101 */
new tagLunarCal( 40, 0, 1, 2, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0 ),
new tagLunarCal( 30, 9, 2, 7, 0, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1 ),
new tagLunarCal( 49, 0, 3, 12, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1 ),
new tagLunarCal( 38, 0, 4, 17, 1, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 0 ), /* 105 */
new tagLunarCal( 27, 6, 6, 23, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 1 ),
new tagLunarCal( 46, 0, 0, 28, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 0 ),
new tagLunarCal( 35, 0, 1, 33, 0, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 1, 0 ),
new tagLunarCal( 24, 4, 2, 38, 0, 1, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1 ), /* 109 */
new tagLunarCal( 42, 0, 4, 44, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1 ),
new tagLunarCal( 31, 0, 5, 49, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0 ),
new tagLunarCal( 21, 2, 6, 54, 0, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1 ),
new tagLunarCal( 40, 0, 0, 59, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1 ), /* 113 */
new tagLunarCal( 28, 6, 2, 5, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 1, 0 ),
new tagLunarCal( 47, 0, 3, 10, 1, 0, 1, 0, 0, 1, 0, 0, 1, 1, 1, 0, 1 ),
new tagLunarCal( 36, 0, 4, 15, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 1, 0, 1 ),
new tagLunarCal( 25, 5, 5, 20, 1, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 1, 0 ), /* 117 */
new tagLunarCal( 43, 0, 0, 26, 1, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1 ),
new tagLunarCal( 32, 0, 1, 31, 1, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 0 ),
new tagLunarCal( 22, 3, 2, 36, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0 ) ]
/* 民国年每月之日数 */
SolarCal = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ]
/* 民国年每月之累积日数, 平年与闰年 */
SolarDays = [ 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365, 396, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366, 397 ]
AnimalIdx = ["马", "羊", "猴", "鸡", "狗", "猪", "鼠", "牛", "虎", "兔", "龙", "蛇" ]
LocationIdx = [ "南", "东", "北", "西" ]
if ( SolarYear <= FIRSTYEAR || SolarYear >LASTYEAR ) return 1
sm = SolarMonth - 1
if ( sm <0 || sm >11 ) return 2
leap = GetLeap( SolarYear )
if ( sm == 1 )
d = leap + 28
else
d = SolarCal[sm]
if ( SolarDate <1 || SolarDate >d ) return 3
y = SolarYear - FIRSTYEAR
acc = SolarDays[ leap*14 + sm ] + SolarDate
kc = acc + LunarCal[y].BaseKanChih
Kan = kc % 10
Chih = kc % 12
Location = LocationIdx[kc % 4]
Age = kc % 60
if ( Age <22 )
Age = 22 - Age
else
Age = 82 - Age
Animal = AnimalIdx[ Chih ]
if ( acc <= LunarCal[y].BaseDays ) {
y--
LunarYear = SolarYear - 1
leap = GetLeap( LunarYear )
sm += 12
acc = SolarDays[leap*14 + sm] + SolarDate
}
else
LunarYear = SolarYear
l1 = LunarCal[y].BaseDays
for ( i=0i<13i++ ) {
l2 = l1 + LunarCal[y].MonthDays[i] + 29
if ( acc <= l2 ) break
l1 = l2
}
LunarMonth = i + 1
LunarDate = acc - l1
im = LunarCal[y].Intercalation
if ( im != 0 &&LunarMonth >im ) {
LunarMonth--
if ( LunarMonth == im ) LunarMonth = -im
}
if ( LunarMonth >12 ) LunarMonth -= 12
today=new Date()
function initArray(){
this.length=initArray.arguments.length
for(var i=0i<this.lengthi++)
this[i+1]=initArray.arguments[i] }
var d=new initArray("星期日","星期一","星期二","星期三","星期四","星期五","星期六")
document.write("<span class=\"jsdhtml\">", today.getYear()," 年 ",today.getMonth()+1," 月 ",today.getDate()," 日 ",d[today.getDay()+1],"")
document.write( "<span class=\"jsdhtml\">【<u>农历 " + LunarMonth + "月" + LunarDate + "日</u>】" )
document.write( "煞" + Location + " 冲" + Animal + Age + "岁</span>" )
return 0
}
/* 求此民国年是否为闰年, 返回 0 为平年, 1 为闰年 */
function GetLeap( year )
{
if ( year % 400 == 0 )
return 1
else if ( year % 100 == 0 )
return 0
else if ( year % 4 == 0 )
return 1
else
return 0
}
function tagLunarCal( d, i, w, k, m1, m2, m3, m4, m5, m6, m7, m8, m9, m10, m11, m12, m13) {
this.BaseDays = d/* 到民国 1 月 1 日到农历正月初一的累积日数 */
this.Intercalation = i /* 闰月月份. 0==此年没有闰月 */
this.BaseWeekday = w /* 此年民国 1 月 1 日为星期几再减 1 */
this.BaseKanChih = k /* 此年民国 1 月 1 日之干支序号减 1 */
this.MonthDays = [ m1, m2, m3, m4, m5, m6, m7, m8, m9, m10, m11, m12, m13 ]/* 此农历年每月之大小, 0==小月(29日), 1==大月(30日) */
}
//-->
CalConv()
</script>
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