js 每个月有多少天算法

function change_date ()
            {
              var month=document.getElementById('start_month');
              
                var  month_temp=month.value.split('月');
               var  year=document.getElementById('start_year') ;
               var year_temp=year.value.split('年');
               var date ;
                if (month_temp[0]=='1'|| month_temp[0]=='3'|| month_temp[0]=='5' || month_temp[0]=="7" || month_temp[0]=="8"  || month_temp[0]=="10"   || month_temp[0]=="12" )
                 {  date=31 ;
                  }
                  if (month_temp[0]=='4' || month_temp[0]=='6'   || month_temp[0]=='9 '  || month_temp[0]=='11' )
                  {
                  date=30;
                  }
                 //  window.alert (month_temp[0]);
                 
                  if (month_temp[0]=='2' )
                  { 
                    if(year_temp[0]%100==0)
                    { 
                      if (year_temp[0]%400==0)
                         {
                           date=29 ;
                         }
                         else
                         {
                        date=28;
                          }
                          
                    } 
                    else
                    {
                       if(((year_temp[0]%100)%4)==0)
                          {
                           date=29 ;
                          }
                          else
                          {  
                            date=28;
                          } 
                     }  
                  }
                  
                    var temp_date =document.getElementById('start_day');
                 for(i=1;i<date;i++)
                 {
                      var add_date=document.createElement("OPTION");
                        add_date.text=i+'日';
                        add_date.value=i+'日';
                        temp_date.add(add_date);
                           
                   }   
                  
            }