获取某一天是该年中的第多少周

判断某一天是该年中的第多少周，需要知道该年的第一天是星期几。

比如第一年第一天是星期一，那么直接获取那一天是该年中的第多少天，取它除以7的上限；

如果不是星期一，则需要进行如下计算：

一周是7天，减去那一天星期几，则该礼拜还有 7 – n天结束，再加一天则是下一礼拜。

示例代码：

   1: //那一年第一天是星期几

   2: var yearFirstDay = new Date(year, 0, 1).getDay() || 7;

3:

   4: var week = null;

   5: if (yearFirstDay == 1) {

   6:     week = Math.ceil(days/yearFirstDay);

   7: } else {

   8:     days -= (7 - yearFirstDay + 1);

   9:     week = Math.ceil(days/7) + 1;

  10:     days = Math.max(days, 1);

  11: }

完整demo代码：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
 <head>
  <title>判定某一天在该年份中是第几周</title>
  <meta name="generator" content="editplus" />
  <meta name="author" content="" />
  <meta name="keywords" content="" />
  <meta name="description" content="" />
  <meta http-equiv='content-type' content='text/html;charset=utf-8'>
 </head>

 <body>
  
<script type="text/javascript">   1:  
   2: /**
   3:  * 判断年份是否为润年
   4:  * 
   5:  * @param {Number} year
   6:  */
   7: function isLeapYear(year) {
   8:     return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
   9: }
  10: /**
  11:  * 获取某一年份的某一月份的天数
  12:  * 
  13:  * @param {Number} year
  14:  * @param {Number} month
  15:  */
  16: function getMonthDays(year, month) {
  17:     return [31, null, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month] || (isLeapYear(year) ? 29 : 28);
  18: }
  19:  
  20:  
  21: function clickHandler() {
  22:     var y = $('year').value,
  23:         m = $('month').value,
  24:         d = $('day').value;
  25:  
  26:     var now = new Date(y, m - 1, d),
  27:         year = now.getFullYear(),
  28:         month = now.getMonth(),
  29:         days = now.getDate();
  30:  
  31:     //那一天是那一年中的第多少天
  32:     for (var i = 0; i < month; i++) {
  33:         days += getMonthDays(year, i);
  34:     }
  35:     
  36:     //那一年第一天是星期几
  37:     var yearFirstDay = new Date(year, 0, 1).getDay() || 7;
  38:  
  39:     var week = null;
  40:     if (yearFirstDay == 1) {
  41:         week = Math.ceil(days/yearFirstDay);
  42:     } else {
  43:         days -= (7 - yearFirstDay + 1);
  44:         week = Math.ceil(days/7) + 1;
  45:  
  46:         days = Math.max(days, 1);
  47:     }
  48:  
  49:     alert(y + "年" + m + "月" + d + "日是" + year + "年的\n\n第" + days + "天\t第" + week + "周");
  50: }
</script>   1:  
   2:  
   3: 选择日期：
   4: <select id="year"></select><label for="year">年</label>
   5: <select id="month"></select><label for="month">月</label>
   6: <select id="day"></select><label for="day">日</label>
   7:  
   8: <button style='margin-left:30px;'>开始计算</button>
   9:  
  10: <script type="text/javascript">
  11: function $(id) {
  12:     return typeof id === 'string' ? document.getElementById(id) : id;
  13: }
  14:  
  15: function addOptions(id, start, end) {
  16:     var opt = null,
  17:         frag = document.createDocumentFragment();
  18:         
  19:     for (var i = start; i <= end ; i++) {
  20:         opt = document.createElement("option");
  21:         opt.value = i;
  22:         opt.innerHTML = i;
  23:         frag.appendChild(opt);
  24:     }
  25:     
  26:     $(id).appendChild(frag);
  27: }
  28:  
  29: function setDays(y, m) {
  30:     addOptions('day', 1, getMonthDays(y, m - 1));
  31: }
  32:  
  33: function changeDays() {
  34:     var val = $('day').value;
  35:     
  36:     $('day').options.length = 0;
  37:  
  38:     var y = $('year').value,
  39:         m = $('month').value;
  40:  
  41:     setDays(y, m);
  42:  
  43:     if (val) {
  44:         var maxDay = getMonthDays(y, m - 1);
  45:  
  46:         $('day').value = (val > maxDay) ? maxDay : val;
  47:     }
  48: }
  49:  
  50: addOptions('year', 1970, 2050);
  51: addOptions('month', 1, 12);
  52: changeDays();
  53:  
  54: //默认设置为本地时间
  55: !(function() {
  56:     var now = new Date();
  57:     $('year').value = now.getFullYear();
  58:     $('month').value = now.getMonth() + 1;
  59:     $('day').value = now.getDate();
  60: })();
  61:  
  62: $('year').onchange = changeDays;
  63: $('month').onchange = changeDays;
  64:  
  65: document.getElementsByTagName("button")[0].onclick = clickHandler;
</script>

 </body>
</html>

在线运行实例：

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
 <head>
  <title>判定某一天在该年份中是第几周</title>
  
  
  
  
  
 </head>

<body>
  
<script type="text/javascript">
/**
 * 判断年份是否为润年
 * 
 * @param {Number} year
 */
function isLeapYear(year) {
	return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}
/**
 * 获取某一年份的某一月份的天数
 * 
 * @param {Number} year
 * @param {Number} month
 */
function getMonthDays(year, month) {
	return [31, null, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month] || (isLeapYear(year) ? 29 : 28);
}

function clickHandler() {
	var y = $('year').value,
		m = $('month').value,
		d = $('day').value;

var now = new Date(y, m - 1, d),
		year = now.getFullYear(),
		month = now.getMonth(),
		days = now.getDate();

//那一天是那一年中的第多少天
	for (var i = 0; i < month; i++) {
		days += getMonthDays(year, i);
	}
	
	//那一年第一天是星期几
	var yearFirstDay = new Date(year, 0, 1).getDay() || 7;

var week = null;
	if (yearFirstDay == 1) {
		week = Math.ceil(days/yearFirstDay);
	} else {
		days -= (7 - yearFirstDay + 1);
		week = Math.ceil(days/7) + 1;

days = Math.max(days, 1);
	}

alert(y + "年" + m + "月" + d + "日是" + year + "年的\n\n第" + days + "天\t第" + week + "周");
}
</script>

选择日期：
<select id="year"></select><label for="year">年</label>
<select id="month"></select><label for="month">月</label>
<select id="day"></select><label for="day">日</label>

function addOptions(id, start, end) {
	var opt = null,
		frag = document.createDocumentFragment();
		
	for (var i = start; i <= end ; i++) {
		opt = document.createElement("option");
		opt.value = i;
		opt.innerHTML = i;
		frag.appendChild(opt);
	}
	
	$(id).appendChild(frag);
}

function setDays(y, m) {
	addOptions('day', 1, getMonthDays(y, m - 1));
}

function changeDays() {
	var val = $('day').value;
	
	$('day').options.length = 0;

var y = $('year').value,
		m = $('month').value;

setDays(y, m);

if (val) {
		var maxDay = getMonthDays(y, m - 1);

$('day').value = (val > maxDay) ? maxDay : val;
	}
}

addOptions('year', 1970, 2050);
addOptions('month', 1, 12);
changeDays();

//默认设置为本地时间
!(function() {
	var now = new Date();
	$('year').value = now.getFullYear();
	$('month').value = now.getMonth() + 1;
	$('day').value = now.getDate();
})();

$('year').onchange = changeDays;
$('month').onchange = changeDays;

document.getElementsByTagName("button")[0].onclick = clickHandler;
</script>

</body>
</html>