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：
判定某一天在该年份中是第几周 选择日期： 年 月 日