(3) 根據選擇周次獲取日期時間範圍

誰有更簡單的方法也告知下，一起參詳參詳。

function getNowFormatDate(theDate){ 
	var day = theDate; 
	var Year = 0; 
	var Month = 0; 
	var Day = 0; 
	var CurrentDate = ""; 
	// 初始化時間 
	Year= day.getFullYear();// ie火狐下都可以 
	Month= day.getMonth()+1; 
	Day = day.getDate(); 
	CurrentDate += Year + "-"; 
	if (Month >= 10) { 
		CurrentDate += Month + "-"; 
	} else { 
		CurrentDate += "0" + Month + "-"; 
	} 
	if (Day >= 10 ){ 
		CurrentDate += Day ; 
	} else { 
		CurrentDate += "0" + Day ; 
	} 
	return CurrentDate; 
} 

//這個方法將取得某年(year)第幾周(weeks)的星期幾(weekDay)的日期 
function getXDate(year,weeks,weekDay){ 
	// 用指定的年構造一個日期物件，並將日期設定成這個年的1月1日 
	// 因為計算機中的月份是從0開始的,所以有如下的構造方法 
	var date = new Date(year,"0","1"); 
	// 取得這個日期物件 date 的長整形時間 time 
	var time = date.getTime(); 
	// 將這個長整形時間加上第N周的時間偏移 
	// 因為第一週就是當前周,所以有:weeks-1,以此類推 
	// 7*24*3600000 是一星期的時間毫秒數,(JS中的日期精確到毫秒) 
	time+=(weeks-1)*7*24*3600000; 
	// 為日期物件 date 重新設定成時間 time 
	date.setTime(time); 
	return getNextDate(date,weekDay); 
} 
// 這個方法將取得 某日期(nowDate) 所在周的星期幾(weekDay)的日期 
function getNextDate(nowDate,weekDay){ 
	// 0是星期日,1是星期一,... 
	weekDay%=7; 
	var day = nowDate.getDay(); 
	var time = nowDate.getTime(); 
	var sub = weekDay-day; 
	if(sub <= 0){ 
		sub += 7; 
	} 
	time+=sub*24*3600000; 
	nowDate.setTime(time); 
	return nowDate; 
}

function isInOneYear(_year,_week){ 
	if(_year == null || _year == '' || _week == null || _week == ''){ 
		return true; 
	} 
	var theYear = getXDate(_year,_week,7).getFullYear(); 
	if(theYear != _year){ 
		return false; 
	} 
	return true; 
} 

// 獲取日期範圍顯示 
function getDateRange(_year,_week){ 
	var beginDate; 
	var endDate; 
	if(_year == null || _year == '' || _week == null || _week == ''){ 
		return ""; 
	} 
	beginDate = getXDate(_year,_week,1); 
	endDate = getXDate(_year,(_week - 0 + 1),7); 
	return getNowFormatDate(beginDate) + " 至 "+ getNowFormatDate(endDate); 
} 
	
//日期處理 
function dateRange(obj){ 	
	//獲取當前年
    var year = $("#yearId").val();
	if(isInOneYear(year,obj.value)){ 
		var showDate = getDateRange(year,obj.value);
		document.getElementById("_dateRange_import").value=showDate;
	}else{ 
		alert(year+"年無"+obj.value+"周，請重新選擇"); 
		return;
	} 
}
 

</pre><pre name="code" class="javascript">/**
 * 獲取當前年份共有多少周次(本次未用到)
 */
function getNumOfWeeks(){
　　var myDate = new Date();
    var year = myDate.getFullYear();  //獲取當前年
　　var d=new Date(year,0,1); 
　　var yt=(( year%4==0 && year%100!=0) || year%400==0)? 366:365;  
　　var allwk = Math.ceil((yt-d.getDay())/7.0);
　　return allwk;
}
 

//獲取當前日期在當前年第幾周函式封裝
function theWeek() {
  var totalDays = 0;
  now = new Date();
  years = now.getYear()
  if (years < 1000)
      years += 1900
  var days = new Array(12);
  days[0] = 31;
  days[2] = 31;
  days[3] = 30;
  days[4] = 31;
  days[5] = 30;
  days[6] = 31;
  days[7] = 31;
  days[8] = 30;
  days[9] = 31;
  days[10] = 30;
  days[11] = 31;
   
  //判斷是否為閏年，針對2月的天數進行計算
  if (Math.round(now.getYear() / 4) == now.getYear() / 4) {
      days[1] = 29
  } else {
      days[1] = 28
  }
  if (now.getMonth() == 0) {
      totalDays = totalDays + now.getDate();
  } else {
      var curMonth = now.getMonth();
      for (var count = 1; count <= curMonth; count++) {
          totalDays = totalDays + days[count - 1];
      }
      totalDays = totalDays + now.getDate();
  }
  //得到第幾周
  var week = Math.ceil(totalDays / 7);//原始碼使用的是round
  return week;
}

//根據選擇年份獲取周次
function getWeeks(obj){
	//年份變更，更改周次對應日期
	$("#_dateRange_import").val("");
	var year = obj.value;
	var d=new Date(year,0,1); 
	var yt=(( year%4==0 && year%100!=0) || year%400==0)? 366:365;  
	var allwk = Math.ceil((yt-d.getDay())/7.0);
	//獲取當前年份，判斷選擇年份是否是當前年份
	var myDate = new Date();
    var curYear = myDate.getFullYear();
    var curweek = document.getElementById("showweeks");
	if(curYear==year){//如果是當前年份，獲取當前日子的周次
		 var theweek = theWeek();
		 var str = "";
		 str+="周次：<select id=\"dateSearch\" onchange=\"dateRange(this)\"><option value=\"\">請選擇周次</option>";
		 for(var i=theweek;i<=allwk;i++){
			 str+="<option value="+i+">第"+i+"周</option>" ;
		 }
		 str+= "</select>"; 
		 curweek.innerHTML = str ;  
	}else{//如果不是獲取年份內的所有周次值
		 var str = "";
		 str+="周次：<select id=\"dateSearch\"onchange=\"dateRange(this)\"><option value=\"\">請選擇周次</option>";
		 for(var i=1;i<=allwk;i++){
			 str+="<option value="+i+">第"+i+"周</option>" ;
		 }
		 str+= "</select>"; 
		 curweek.innerHTML = str ;  
	}
}
 

<jsp:useBean id="today" class="java.util.GregorianCalendar" /> 

<table>
	<tr>
		<td>
			年份 : 
			<select id="yearId" onchange="getWeeks(this)" >
				<c:forEach begin="${today.get(1)}" end="2020" var="i" step="1">
					<option value="<c:out value='${i}'/>"><c:out value='${i}'/>年</option>
				</c:forEach>	
			</select> 
		</td>
		<td id="showweeks">
			周次：
			<select id="dateSearch" onchange="dateRange(this)">
				<option value="">請選擇周次</option>
				<c:forEach begin="${today.get(3)}" end="52" var="i" step="1">
					<option value="<c:out value='${i}'/>">第<c:out value='${i}'/>周</option>
				</c:forEach>	
		</select> 
		</td>
		<td>
			時間段：<input id ="_dateRange_import" readonly="readonly" type="text" />
		</td>
	</tr>
</table>

原文連結： js獲取指定日期週數以及星期幾的小例子，這個是從每週的週四開始計算的。