JSFiddle - React, Tailwind, and code Playground

HTML

<form id="ad_form" method="post" action="">
    <p>
        <select id="ad_type" name="ad_type">
            <option value="" selected="selected">Select premium ad type</option>
            <option value="<?php echo TYPE_USER;?>">Featured Agent</option>
            <option value="<?php echo TYPE_LISTING;?>">Featured Listing</option>
        </select>
        <label for="ad_type" class="labelStrong">Advertising Type</label>
    </p>
    <p>
        <select id="ad_duration" name="ad_duration">
            <option value="" selected="selected">Select premium ad duration</option>
            <option value="weekly">Weekly</option>
            <option value="fortnightly">Fortnightly</option>
            <option value="monthly">Monthy</option>
        </select>
        <label for="ad_duration" class="labelStrong">Advertising Duration</label>
    </p>

    <p>
        <div id="calender">
        </div>
    </p>
    <p>
        <input type="submit" name="submit" value="Submit" id="submitorder" />
    </p>
</form>

JavaScript

$('#submitorder').attr('disabled', 'disabled');

function updateFormEnabled() {
    if (verifyAdSettings()) {
        $('#submitorder').attr('disabled', '');
    } else {
        $('#submitorder').attr('disabled', 'disabled');
    }
}

function verifyAdSettings() {
    if ($('#ad_duration').val() != '' && $('#ad_type').val() != '') {
        return true;
    } else {
        return false
    }
}

$('#ad_duration').change(updateFormEnabled);

$('#ad_type').change(updateFormEnabled);