How to fix timezone issue.

The Author seems to have given up on this plugin, so I’ll put my solution in case it helps others.

Open the main plugin file /wp-content/plugins/wp-countdown-timer/wordpress-countdown-timer.php

Replace the lines 69-76 with the following :
(remember to use your own timezone)

function countdown_timer_shortcode(){
	// Set this to your timezone:
	// https://php.net/manual/en/timezones.php
	date_default_timezone_set('Australia/Brisbane');
	$date_to = get_option('countdown-timer-date');
	$date_from = date('m/d/Y h:i:s');
	//echo $date_from."";
	//echo $date_to;die;
	$date_to = new DateTime($date_to);
	$date_from = new DateTime();
	$date_diff = $date_from->diff($date_to,false);