JSFiddle - React, Tailwind, and code Playground

HTML

<select name="month1" id="month1" size="1">
    <option value="">Choose a Month</option>
    <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 name="month2" id="month2" size="1">
    <option value="">Choose an Option</option>
    <option value="2">2</option>
    <option value="14">14</option>
</select>
<select name="month3" id="month3" size="1">
    <option value="">Choose an Option</option>
    <option value="3">3</option>
    <option value="15">15</option>
</select>

JavaScript

(function() {
    var monthsToGo;

    $(document).ready(function() {
        $('#month2, #month3').hide();
    });
    
    $('#month1').on('change', function() {
        // ignore blank options
        if ($(this).val() == '') { return; }
        
        var today = new Date();
        var birthdate = new Date();
        birthdate.setMonth($('#month1').val());

        monthsToGo = (today.getMonth() < birthdate.getMonth())
            ? birthdate.getMonth() - today.getMonth()
            : (12 - today.getMonth()) + birthdate.getMonth()
        ;
        
        $('#month2').hide();
        $('#month3').hide();

        if (monthsToGo == 1) {
            this.monthsToGo = 13;
            alert(this.monthsToGo);
        } else if (monthsToGo == 2) {
            $('#month2').show();
        } else if (monthsToGo == 3) {
            $('#month3').show();
        } else {
            this.monthsToGo = monthsToGo;
            alert(this.monthsToGo);
        }
    });

    $('#month2, #month3').on('change', function() {
        this.monthToGo = $(this).val();
        alert(this.monthToGo);
    });
}($))