//判斷一天是否是年月中正確的日
   function IsDayRight(text,columnname)
   {  
       if(document.all[text].value=="")
       {
           doFlag_errObj(text,true);
           return true;
       }
 
       if(document.all[text].value.length==1)
       {
           if(document.all[text].value=="0")
            {
               document.all[text].value ="0"
            }
            else
            {
               document.all[text].value ="0"+document.all[text].value;
            }
       }
       if(document.all['txtYYMM'].value.substring(5,7).charAt(0)=="0")
       {
           strMonth = parseInt(document.all['txtYYMM'].value.substring(5,7).replace("0",""));
       }
   
       var strText = parseInt(document.all[text].value);
       var strYear = parseInt(document.all['txtYYMM'].value.substring(0,4));
       var strMonth = parseInt(document.all['txtYYMM'].value.substring(5,7));
       if(document.all[text].value.charAt(0)=="0")
       {
           strText = parseInt(document.all[text].value.replace("0",""));
       }
       if(onblur_IsYYYYMM('txtYYMM','銷貨年月',false))
       {
           if((!check_IsNumber(document.all[text].value)) || strText<1 || strText>DaysOfMonth(strMonth,strYear) || document.all[text].value =="0")
           {
               document.all[text].focus();
                  doFlag_errObj(text,false);
                  document.all[text].select();
               alert(columnname + " 需輸入正確年月中的日!!\n");
               return false;
           }
           else
           {
               doFlag_errObj(text,true);
                  return true;
           }
       }
   }
   
   
   
   //判斷閏年
   function IsLeap(year)     
   {
       if((year%4==0 && year%100!=0) || (year%400==0))
       {
           return true;
       }
       else
       {  
           return false;
       }
            }
            //判斷一年中一個月又多少天
            function DaysOfMonth(month,year)
            {
                switch(month)
                {
                case 1:
                case 3:
                case 5:
                case 7:
                case 8:
                case 10:
                case 12:
                    return 31;
                case 2:
                    if(IsLeap(year))
                        return 29;
                    else
                        return 28;
                default:
                    return 30;
                }
            }

使用方法：onblur="IsDayRight('txtDAY3','第三週日止(含)')"
代碼寫的好濫