Javascript: Age validation with jquery.validate.js #snippet

ageCheck.js

$.validator.addMethod("checkage", function(value, element, params) {
    var subDate, userDate, today, diff;
    subDate = getSubmittedDate(); // some function that returns the user's birthdate as a Date
    userDate = new Date((subDate.getFullYear() + params), subDate.getMonth(), subDate.getDate());
    today = new Date();
    diff = today.getTime() - userDate.getTime();
    return diff > -1;
}, "You must be over " + params.toString() + " to enter");

$('#my-form').validate({
    groups: {
        dob: "age_dob_month age_dob_day age_dob_year age_dob"
    },
    rules: {
        age_dob_month: "required",
        age_dob_day: "required",
        age_dob_year: "required",
        age_dob: {
            required: true,
            checkage: 21
        }
    }
}

form.html

<!-- SNIP -->
<div id="dob-input">
    <label for="dob_month">Birthdate</label>
    <select id="dob_month" name="dob_month">
        <option value="">Month</option>
    </select>
    <select id="dob_day" name="dob_day">
        <option value="">Day</option>
    </select>
    <select id="dob_year" name="dob_year">
        <option value="">Year</option>
    </select>
    <div class="hidden-field"><input id="dob" name="dob" type="text"/></div>
</div>

style.css

.hidden-field input {
    visibility: hidden;
    height: 0;
}

.hidden-field {
    width: auto;
}