Jetpack::is_development_version PHP Method

    public static function is_development_version()
    {
        /**
         * Allows filtering whether this is a development version of Jetpack.
         *
         * This filter is especially useful for tests.
         *
         * @since 4.3.0
         *
         * @param bool $development_version Is this a develoment version of Jetpack?
         */
        return (bool) apply_filters('jetpack_development_version', !preg_match('/^\\d+(\\.\\d+)+$/', Jetpack_Constants::get_constant('JETPACK__VERSION')));
    }

function twentyfifteen_init_jetpack()
{
    /**
     * Add our compat CSS file for custom widget stylings and such.
     * Set the version equal to filemtime for development builds, and the JETPACK__VERSION for production.
     */
    $version = Jetpack::is_development_version() ? filemtime(plugin_dir_path(__FILE__) . 'twentyfifteen.css') : JETPACK__VERSION;
    wp_enqueue_style('twentyfifteen-jetpack', plugins_url('twentyfifteen.css', __FILE__), array(), $version);
    wp_style_add_data('twentyfifteen-jetpack', 'rtl', 'replace');
}