javascript取当月第一，最后一天，本周第一，最后一天

javascript取当月第一，最后一天，本周第一，最后一天
<script   language=javascript>  
  function   showWeekFirstDay()  
  {  
  var   Nowdate=new   Date();  
  var   WeekFirstDay=new   Date(Nowdate-(Nowdate.getDay()-1)*86400000);  
  alert(WeekFirstDay)  
  }  
  function   showWeekLastDay()  
  {  
  var   Nowdate=new   Date();  
  var   WeekFirstDay=new   Date(Nowdate-(Nowdate.getDay()-1)*86400000);  
  var   WeekLastDay=new   Date((WeekFirstDay/1000+6*86400)*1000);  
  alert(WeekLastDay)  
  }  
  function   showMonthFirstDay()  
  {  
  var   Nowdate=new   Date();  
  var   MonthFirstDay=new   Date(Nowdate.getYear(),Nowdate.getMonth(),1);  
  alert(MonthFirstDay)  
  }  
  function   showMonthLastDay()  
  {  
  var   Nowdate=new   Date();  
  var   MonthNextFirstDay=new   Date(Nowdate.getYear(),Nowdate.getMonth()+1,1);  
  var   MonthLastDay=new   Date(MonthNextFirstDay-86400000);  
  alert(MonthLastDay)  
  }  
  </script>