JavaScript operation of adding days, months, years, and weeks to existing dates

<form name="form">

<TABLE>
<TR>
    <TD>现在的日期：</TD>
    <TD><input type="text" name="u" value="2009-01-01"></TD>
</TR>
<TR>
    <TD>要加的天数：</TD>
    <TD><INPUT TYPE="text" name="entry" id="d" VALUE="" onkeyup="calculate('d')"></TD>
</TR>
<TR>
    <TD>要加的周数：</TD>
    <TD><INPUT TYPE="text" name="entry" id="w" VALUE="" onkeyup="calculate('w')"></TD>
</TR>
<TR>
    <TD>要加的月数：</TD>
    <TD><INPUT TYPE="text" name="entry" id="m" VALUE="" onkeyup="calculate('m')"></TD>
</TR>
<TR>
    <TD>要加的年数：</TD>
    <TD><INPUT TYPE="text" name="entry" id="y" VALUE="" onkeyup="calculate('y')"></TD>
</TR>
<TR>
    <TD>结果：</TD>
    <TD><input name="twice"></TD>
</TR>
<!--<TR>
    <TD>要加的日期2：</TD>
    <TD><input type="text" name="u" value="2009/01/01"></TD>
</TR>
<TR>
    <TD>结果2：</TD>
    <TD><input name="twice"></TD>
</TR>-->
</TABLE>
</form>
<script language="JavaScript">
function   DateAdd(strInterval,   NumDay,   dtDate)   {  
  var   dtTmp   =   new   Date(dtDate);  
  if   (isNaN(dtTmp))   dtTmp   =   new   Date();  
  switch   (strInterval)   {  
      case   "s":
           dtTmp  =   new   Date(Date.parse(dtTmp)   +   (1000   *   parseInt(NumDay)));
           break; 
      case   "n":
           dtTmp  =   new   Date(Date.parse(dtTmp)   +   (60000   *   parseInt(NumDay)));
           break; 
      case   "h":
           dtTmp  =   new   Date(Date.parse(dtTmp)   +   (3600000   *   parseInt(NumDay)));
           break;
      case   "d":
           dtTmp  =   new   Date(Date.parse(dtTmp)   +   (86400000   *   parseInt(NumDay)));
           break;
      case   "w":
           dtTmp  =   new   Date(Date.parse(dtTmp)   +   ((86400000   *   7)   *   parseInt(NumDay)));
           break;
      case   "m":
           dtTmp  =   new   Date(dtTmp.getFullYear(),   (dtTmp.getMonth())+parseInt(NumDay),   dtTmp.getDate(),   dtTmp.getHours(),   dtTmp.getMinutes(),   dtTmp.getSeconds());
           break;  
      case   "y":
           //alert(dtTmp.getFullYear());
           dtTmp  =   new   Date(dtTmp.getFullYear()+parseInt(NumDay),   dtTmp.getMonth(),   dtTmp.getDate(),   dtTmp.getHours(),   dtTmp.getMinutes(),   dtTmp.getSeconds());
           //alert(dtTmp);
           break;
  }
  var mStr=new String(dtTmp.getMonth()+1);
  var dStr=new String(dtTmp.getDate());
  if (mStr.length==1){
      mStr="0"+mStr;
  }
  if (dStr.length==1){
      dStr="0"+dStr;
  }
  return dtTmp.getFullYear()+"-"+mStr+"-"+dStr;
}  
</script>
<script language="JavaScript">

function calculate(type)
{
    var entry = document.getElementById(type).value;//自己判断是否合法
    var uValue=document.form.u.value;
    uValue=uValue.replace("-","/").replace("-","/");
    //alert(uValue);
    document.form.twice.value = DateAdd(type,entry,uValue);
}
</script>