紧急求助大家关于用Javascript计算日期的问题，谢谢

不用那么麻烦的,直接加7个月即可
var today = new Date();
var d = new Date(today.getFullYear(), today.getMonth() + 7, today.getDate()-1);
alert(d.getFullYear() + "-" + (d.getMonth()+1) + "-" + d.getDate());

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随意日期也是一样的
var theday = new Date(年,月-1, 日);
2007-06-01的话
var theday = new Date(2007,5,1);
var d = new Date(theday.getFullYear(), theday.getMonth() + 12, theday.getDate()-1);
d就是2008-05-31了.
2007-06-01拆分年月日也简单
a="2007-06-01".split("-");
a[0] = parseInt(a[0]); //2007
a[1] = parseInt(a[1]); //6
a[2] = parseInt(a[2]); //1
<script language="JavaScript" >
function youxiaoriqi()
{
  var jiandingriqi=document.form1.jdrq.value;
  var sel = document.form1.jdzq;
  if(sel.selectedIndex==0 || jiandingriqi=="") return false;
  var a = jiandingriqi.split("-");
  if(a.length != 3) return false;
  var theday = new Date(parseInt(a[0]),parseInt(a[1]-1),parseInt(a[2]));
  var d = new Date(theday.getFullYear(), theday.getMonth() + parseInt(sel.options[sel.selectedIndex].value), theday.getDate()-1);
  var m = parseInt(d.getMonth())+1;
  var t =  d.getDate();
  m = "0" + m;
  m = m.substr(m.length-2,2);
  t = "0" + t;
  t = t.substr(t.length-2,2);
  document.form1.yxrq.value = d.getFullYear() + "-" + m + "-" + t;
}
</script> <style type="text/css">

</style><form name="form1" method="post" action="Untitled-2-1.asp" >
  <table width="100%" border="0" cellspacing="1" cellpadding="2">
    <tr bgcolor="#4E5960">
      <td height="20" align="center" class="heading"><font color="#FFFFFF" class="STYLE2"><b>证书 / 报告录入</b></font></td>
    </tr>
  </table>
<table width="100%" border="1" cellspacing="0" cellpadding="1" bordercolordark="#FFFFFF" bordercolor="#000000">
  <tr>
      <td width="25%" bgcolor="#bfbfbf">
      <div align="center" class="STYLE2">检定日期：</div>    </td>
      <td width="75%" bgcolor="#efefef">
        <input name="jdrq" type="text" class="STYLE2" value=""size="20" maxlength="20" /></td>
  </tr>
  <tr>
      <td width="25%" height="30" bgcolor="#bfbfbf">
      <div align="center" class="STYLE2">检定周期：</div>    </td>
      <td width="75%" bgcolor="#efefef" style="height:30px;position:relative">
  <select name="jdzq" class="STYLE2" id="jdzq1" onChange="youxiaoriqi()">
        <option value="">选择检定周期：</option>
        <option value="2">2</option>
        <option value="3">3</option>
        <option value="6">6</option>
        <option value="12">12</option>
        <option value="24">24</option>
        <option value="36">36</option>
      </select>
      </td>
  </tr>
  <tr>
      <td width="25%" bgcolor="#bfbfbf">
      <div align="center" class="STYLE2">有效日期：</div>    </td>
      <td width="75%" bgcolor="#efefef">
      <input name="yxrq" type="text" class="STYLE2" size="20" maxlength="20" />    </td>
  </tr></table>
</form>
<script language="JavaScript" >
function youxiaoriqi()
{
  var jiandingriqi=document.form1.jdrq.value;
  var sel = document.form1.jdzq;
  if(sel.selectedIndex==0 || jiandingriqi=="") return false;
  var theday =  new Date(jiandingriqi.replace(/-/ig,"/"));
  var d = new Date(theday.getFullYear(), theday.getMonth() + parseInt(sel.options[sel.selectedIndex].value), theday.getDate()-1);
  if(d.getDate()+1 != theday.getDate()) d.setDate(0);
  var m = parseInt(d.getMonth())+1;//加了这一句
  var t =  d.getDate();
  m = "0" + m;
  m = m.substr(m.length-2,2);
  t = "0" + t;
  t = t.substr(t.length-2,2);
  document.form1.yxrq.value = d.getFullYear() + "-" + m + "-" + t;
}
</script> <head>
<style type="text/css">

</style><form name="form1" method="post" action="Untitled-2-1.asp" >
</head>  <table width="100%" border="0" cellspacing="1" cellpadding="2">
    <tr bgcolor="#4E5960">
      <td height="20" align="center" class="heading"><font color="#FFFFFF" class="STYLE2"><b>证书 / 报告录入</b></font></td>
    </tr>
  </table>
<table width="100%" border="1" cellspacing="0" cellpadding="1" bordercolordark="#FFFFFF" bordercolor="#000000">
  <tr>
      <td width="25%" bgcolor="#bfbfbf">
      <div align="center" class="STYLE2">检定日期：</div>    </td>
      <td width="75%" bgcolor="#efefef">
        <input name="jdrq" type="text" class="STYLE2" value="2007-12-31" size="20" maxlength="20"   /></td>
  </tr>
  <tr>
      <td width="25%" height="30" bgcolor="#bfbfbf">
      <div align="center" class="STYLE2">检定周期：</div>    </td>
      <td width="75%" bgcolor="#efefef" style="height:30px;position:relative">
  <select name="jdzq" class="STYLE2" id="jdzq1" onChange="youxiaoriqi()">
        <option value="">选择检定周期：</option>
        <option value="2">2</option>
        <option value="3">3</option>
        <option value="6">6</option>
        <option value="12">12</option>
        <option value="24">24</option>
        <option value="36">36</option>
      </select>
      </td>
  </tr>
  <tr>
      <td width="25%" bgcolor="#bfbfbf">
      <div align="center" class="STYLE2">有效日期：</div>    </td>
      <td width="75%" bgcolor="#efefef">
      <input name="yxrq" type="text" class="STYLE2" size="20" maxlength="20" />    </td>
  </tr></table>
</form>
sorry,  "//加了这一句"写错了地方，应该是上一句
改一下hookee的函数:
function youxiaoriqi()
{
    var d = new Date(document.getElementById("jdrq").value.replace(/-/g,"/"));
    var zq = document.getElementById("jdzq").value;
    if(!d || zq =="") return false;
    d.setMonth(d.getMonth() -(0-zq) );
    d.setDate(d.getDate()-1);
    var str = (d.getFullYear()+"-"+(d.getMonth()+1)+"-"+d.getDate()).replace(/(\D)(\d)(?=\D|$)/g, "$10$2");
    document.getElementById("yxrq").value = str;
}
看不出要转什么弯？
给input加个onblur事件就成。简单的事自己完成，
因为别人也不想当代码工。