JSFiddle - React, Tailwind, and code Playground

HTML

<select name="month1" id="month1" size="1">
    <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>

JavaScript

$('#month1').on('change', function() {
    var today = new Date();
    var birthdate = new Date();
    birthdate.setMonth($('#month1').val());

    alert(today.getMonth() - birthdate.getMonth());
});