you could do it after query with php with code like that:
<?php while($event->have_posts()) : $event->the_post();
$today = date('Ymd');
$event_date = get_field('date');
if (strtotime($today) <= strtotime($event_date)) {
?>
<li>
Posted on: <?php the_field('date'); ?><a href="<?php the_permalink() ?>"><?php the_title(); ?></a>
</li>
<?php } endwhile; ?>
else you need to work with meta_value that is always build like 20150430 year month day without spaces, no matter what input or output format you give the date field with acf settings for that field.
