Help with custom query with custom fields

I have a query to grab certain posts based on a custom field (a date). The query is supposed to display posts with the custom field date in the future. The problem is that the custom field is stored as m/d/Y rather than Y/m/d. This is an issue when comparing dates because it doesn’t use the year to base if it is in the future or not.

So, what I need to do, is to somehow read the custom field, change the format and then compare the dates.

My current query

<?php
     $paged = ( get_query_var('paged') ) ? get_query_var('paged') : 1;
     $today = date('Y/m/d', strtotime('-6 hours'));
     $myquery = new WP_Query(array(
     'post_type' => 'itineraries',
     'posts_per_page' => 20,
     'paged' => $paged,
	 'meta_key' => 'start_date',
     'orderby' => 'meta_value',
     'order' => 'ASC',
     'tax_query' => array(
		array(
			'taxonomy' => 'regions',
			'field' => 'slug',
			'terms' => 'future'
		)
	)
    ));

    if ($myquery->have_posts()) :
    while ($myquery->have_posts()) : $myquery->the_post();
?>

So above, I need to convert the meta_key start_date to the Y/m/d format for better comparison. How do I do this using code?

I did some research and implode(‘-’, array_reverse(explode(‘/’,$date))); seems like it might be helpful but I’m not sure how to incorporate it in.

Thanks for any help