//獲取最近7天日期
getDay(0);//當天日期
getDay(-7);//7天前日期

//獲取最近3天日期
getDay(0);//當天日期
getDay(-3);//3天前日期

function getDay(day){  
       var today = new Date();  
         
       var targetday_milliseconds=today.getTime() + 1000*60*60*24*day;          
  
       today.setTime(targetday_milliseconds); //注意，這行是關鍵程式碼
         
       var tYear = today.getFullYear();  
       var tMonth = today.getMonth();  
       var tDate = today.getDate();  
       tMonth = doHandleMonth(tMonth + 1);  
       tDate = doHandleMonth(tDate);  
       return tYear+"-"+tMonth+"-"+tDate;  
}  
function doHandleMonth(month){  
       var m = month;  
       if(month.toString().length == 1){  
          m = "0" + month;  
       }  
       return m;  
}