WordPress - Add filter to 'event' post type to filter events in the past/future (given by event_date meta field)

gistfile1.php

<?php

// Add past/future events filter
add_action( 'restrict_manage_posts', 'my_add_past_prev_filter' );
function my_add_past_prev_filter() {
	global $typenow;
	// get our time period filter (if applied)
	$time_period = $_GET['time_period'];
	
	// set our select options
	$future_option = '<option value="future">Future events</option>';
	$past_option = '<option value="past">Past events</option>';
	
	// test for selected option
	if($time_period=='future') {
		$future_option = '<option value="future" selected="selected">Future events</option>';
	}
	elseif($time_period=='past') {
		$past_option = '<option value="past" selected="selected">Past events</option>';
	}
	
	// output additional filter
	if( $typenow != "post" && $typenow != "page" && $typenow == "event" ) {
		echo '<select name="time_period" id="time_period" class="postform">';
		echo '<option value="all">All events</option>';
		echo $future_option;
		echo $past_option;
		echo '</select>';
	}
}

// If filter is set, change the query
add_filter( 'parse_query', 'sort_events_by_past_prev_filter' );
function sort_events_by_past_prev_filter($query) {
    global $pagenow;
	// get our time period filter (if applied)
	$time_period = $_GET['time_period'];
    if (is_admin() && $pagenow=='edit.php' &&
        isset($_GET['post_type']) && $_GET['post_type']=='event' && 
        isset($_GET['time_period']) && $_GET['time_period'] !='all')  {
        
        // amend the query
        if($time_period=='future') {
	        $query->query_vars['meta_key'] = 'event_date';
	        $query->query_vars['meta_value'] = date("Y-m-d");
	        $query->query_vars['meta_compare'] = '>=';
        }
        elseif($time_period=='past') {
	        $query->query_vars['meta_key'] = 'event_date';
	        $query->query_vars['meta_value'] = date("Y-m-d");
	        $query->query_vars['meta_compare'] = '<=';
        }
        
    }
}

?>