JavaScript 实现获取当前周、上一周及下一周日期的方法

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html >
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  <title>日历操作</title>
  <script>
    window.onload = function(){     
      var cells = document.getElementById('monitor').getElementsByTagName('td');
      var clen = cells.length;
      var currentFirstDate;
      var formatDate = function(date){       
        var year = date.getFullYear()+'年';
        var month = (date.getMonth()+1)+'月';
        var day = date.getDate()+'日';
        var week = '('+['星期天','星期一','星期二','星期三','星期四','星期五','星期六'][date.getDay()]+')'; 

        return year+month+day+' '+week;
      };
      var addDate= function(date,n){    
        date.setDate(date.getDate()+n);    
        return date;
      };
      var setDate = function(date){       
        var week = date.getDay()-1;
        date = addDate(date,week*-1);
        currentFirstDate = new Date(date);

        for(var i = 0;i<clen;i++){         
          cells[i].innerHTML = formatDate(i==0 ? date : addDate(date,1));
        }        
      };       
      document.getElementById('last-week').onclick = function(){
        setDate(addDate(currentFirstDate,-7));     
      };       
      document.getElementById('next-week').onclick = function(){         
        setDate(addDate(currentFirstDate,7));
      };   
      setDate(new Date());
    }
  </script>
</head>
<body>
  <button id="last-week">上一周</button><button id="next-week">下一周</button>
  <table id="monitor">
    <tr>
      <td></td>
      <td></td>
      <td></td>
      <td></td>
      <td></td>
      <td></td>
      <td></td>
    </tr>
  </table>
</body>
</html>