pre_get_posts and custom field values

Hi

I am trying to use pre_get_posts to alter a query for a custom post type. I would like to order the results by the values of a custom field. My custom field is called “event_start_date” and a typical value is “02/12/2012 19:00”. The format of the string is dd/mm/yy h:i

Is this possible using pre_get_posts and custom field values?

The trouble I have is that the custom field holds this type of string value, and not a proper time stamp. So doing comparisons etc is difficult.

Here is what I have cobbled together so far. However it is not ordering these posts by their ‘date’ order…

function my_event_query( $query ) {

	if( is_post_type_archive( 'events' ) ) {
		$meta_query = array(
			array(
				'key' => 'event_start_date',
				'value' => time(),
				'compare' => '>'
			)
		);
		$query->set( 'meta_query', $meta_query );
		$query->set( 'orderby', 'meta_value_num' );
		$query->set( 'meta_key', 'event_start_date' );
		$query->set( 'order', 'ASC' );
		$query->set( 'posts_per_page', '-1' );
	}

}

add_action( 'pre_get_posts', 'my_event_query' );

Is there any possibility of extracting the day/month/year time components so that I can order by date and time?

Thanks in advance

Rich