某年某月有多少天

 <script type="text/javascript">
    function MonthOfDay()
    {
       var year=document.getElementById("Tb_year").value;
       if(year.length!=4)
       {
          alert('请输入正确的年份！');
          return;

       }
       if(isNaN(year)==true)
       {
          alert('年份必须为数字');
          return;

       }
       var month=document.getElementById("Tb_month").value;
       if(month>12 || month<=0)
       {
          alert('请输入正确的月份！');
          return;

       }
       if(isNaN(month)==true)
       {
          alert('月份必须为数字');
          return;

       }
      
       if((year!="")&&(month!=""))
       {
          switch(month)
          {
             case '1':
             case '3':
             case '5':
             case '7':
             case '9':
             case '10':
             document.getElementById("Tb_Day").value=31;
             break;
            
             case '4':
             case '6':
             case '8':
             case '11':
             case '12':
             document.getElementById("Tb_Day").value=30;
             break;
            
             case '2':
             if((year%4==0)&&(year%100!=0)||(year%400==0))
             {
                document.getElementById("Tb_Day").value=29;
             }
             else
             {
                document.getElementById("Tb_Day").value=28;
             }
             break;
            
             default:
             break;
          };
       }
       else
       {
          alert('请输入要查询的年份或月份!');
       }
      
    }
    </script>