[jQuery] jQuery validation on 3 select box

[c.sokun]

Hi there,

I had a form where I need user to input the their birthdate:

<select id="sel_dd" name="sel_dd">
 <option value=""></option>
 <option value="1">1</option>
...
</select>

<select id="sel_mm" name="sel_mm">
 <option value=""></option>
 <option value="1">Jan</option>
...
</select>

<select id="sel_yy" name="sel_yy">
 <option value=""></option>
 <option value="1980">1980</option>
...
</select>

How do I write custom validation rule to check if user had selected
appropriate input?
And which control should I assign the rule on?

Thanks.