MODX - Register - DOB done on Frontend

Not a full script - pieces of example

<section>
            <label>Birth Date </label>
            <select id="birthMonth">
                <option value="0">January</option>
                                <option value="1">February</option>
                                <option value="2">March</option>
                                <option value="3">April</option>
                                <option value="4">May</option>
                                <option value="5">June</option>
                                <option value="6">July</option>
                                <option value="7">August</option>
                                <option value="8">September</option>
                                <option value="9">October</option>
                                <option value="10">November</option>
                                <option value="11">December</option>
            </select>
            <select id="birthDay">
                <option value="01">01</option>
                <option value="02">02</option>
                <option value="03">03</option>
                <option value="04">04</option>
                <option value="05">05</option>
                <option value="06">06</option>
                <option value="07">07</option>
                <option value="08">08</option>
                <option value="09">09</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>
                <option value="31">31</option>
            </select>
            <select id="birthYear">
                <option value="">Select</option>
                                <option value="2014">2014</option>
                                <option value="2013">2013</option>
                                <option value="2012">2012</option>
                                <option value="2011">2011</option>
                                <option value="2010">2010</option>
                                <option value="2009">2009</option>
                                <option value="2008">2008</option>
                                <option value="2007">2007</option>
                                <option value="2006">2006</option>
                                <option value="2005">2005</option>
                                <option value="2004">2004</option>
                                <option value="2003">2003</option>
                                <option value="2002">2002</option>
                                <option value="2001">2001</option>
            </select>
            <input type="hidden" name="dob" id="dob" value="">
  </section>




<script>
    $('#birthYear,#birthDay,#birthMonth').on('change', function(){
		//inputs
		var birth_month = $('#birthMonth').val();
	  var birth_day = $('#birthDay').val();
	  var birth_year = $('#birthYear').val();
		//create date
		var theBirthDay = new Date();
		    theBirthDay.setMonth(birth_month,birth_day);
		    theBirthDay.setFullYear(birth_year);
		var formattedDate = (Date.parse(theBirthDay)/1000).toFixed(0);   
		//formattedDate - 2009-07-27  ==   1248716903
		//MODX DB - 2009-07-27        ==   1248670800
		$("#dob").val(formattedDate);
	});
</script>