<html>
<body>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<select name='years' id='years' onchange="updateDays()">
<option value ='0'>0</option>
<option value ='1'>1</option>
<option value ='2'>2</option>
<option value ='3'>3</option>
<option value ='4'>4</option>
<option value ='5'>5</option>
</select>年
<select name='months' id='months' onchange="updateDays()">
<option value ='0'>0</option>
<option value ='1'>1</option>
<option value ='2'>2</option>
<option value ='3'>3</option>
<option value ='4'>4</option>
<option value ='5'>5</option>
<option value ='6'>6</option>
<option value ='7'>7</option>
<option value ='8'>8</option>
<option value ='9'>9</option>
<option value ='10'>10</option>
<option value ='11'>11</option>
<option value ='12'>12</option>
</select>月
<select name='days' id='days' onchange="updateDays()">
<option value ='0'>0</option>
<option value ='1'>1</option>
<option value ='2'>2</option>
<option value ='3'>3</option>
<option value ='4'>4</option>
<option value ='5'>5</option>
<option value ='6'>6</option>
<option value ='7'>7</option>
<option value ='8'>8</option>
<option value ='9'>9</option>
<option value ='10'>10</option>
<option value ='11'>11</option>
<option value ='12'>12</option>
<option value ='13'>13</option>
<option value ='14'>14</option>
<option value ='15'>15</option>
<option value ='16'>16</option>
<option value ='17'>17</option>
<option value ='18'>18</option>
<option value ='19'>19</option>
<option value ='20'>20</option>
<option value ='21'>21</option>
<option value ='22'>22</option>
<option value ='23'>23</option>
<option value ='24'>24</option>
<option value ='25'>25</option>
<option value ='26'>26</option>
<option value ='27'>27</option>
<option value ='28'>28</option>
<option value ='29'>29</option>
<option value ='30'>30</option>
</select>日
<br><br><br>
<input type="text" name="validity" id="validity" >
<script language="javascript">
function updateDays(){
//接受年月日
var years = $("#years").val();
//如果年为5则月和日为0
if(years==5){
$("#months").val("0");
$("#days").val("0");
}
var months = $("#months").val();
var days = $("#days").val();
//计算总天数
var allDays = (years*365)+(months*30)+days*1;
$("#validity").val(allDays);
}
</script>
</body>
</html>
来源：http://www.cnblogs.com/zhangyiqing/archive/2012/01/06/2315074.html