[jQuery] Jquery Validation on Dropdown list Question

How can I validate this as one hidden field.     I have 3 dropbdown list
month, day, year and I do not want to validate each separate on so I
would like to combine the strings together and validate the hidden
field. Pleas and thank you for your help.

        <fieldset id="section-dob" class="group">
         <legend><span>Birth date</span></legend>
         
         <div>
         <label for="dob_month">Month</label>
         <select name="month" value="" id="month"
            onchange="document.getElementById
'birthdate'
.value =
                document.getElementById
'month'
.value
'/'

document.getElementById
'day'
.value
'/'

document.getElementById
'year'
.value;
            ">
         <option selected="selected" value="" >Month</option>
         <?php
for
$i=1;$i<=12;$i
{
echo "<option>" . $i . "</option>";
}
?>
     </select>
     </div>
         
         <div>
         <label for="dob_day">Day</label>
         <select name="day" value="" id="day"
            onchange="
                document.getElementById
'birthdate'
.value =
                document.getElementById
'month'
.value
'/'

document.getElementById
'day'
.value
'/'

document.getElementById
'year'
.value;
            ">
<option selected="selected" value="">Day</option>
<?php
for
$i=1;$i<=31;$i
{
echo "<option>" . $i . "</option>";
}
?>
</select>
     </div>
         
         <div>
         <label for="dob_year">Year</label>
         <select name="year" value="" id="year"
            onchange="
                document.getElementById
'birthdate'
.value =
                document.getElementById
'month'
.value
'/'

document.getElementById
'day'
.value
'/'

document.getElementById
'year'
.value;
            ">
<option selected="selected" value="">Year</option>
<?php
for
$i=2005;$i<=2008;$i
{
echo "<option>" . $i . "</option>";
}
?>
</select>