Wordpress sort by meta key on date archive

pre_get_posts_on_date_archive.php

<?php

add_action( 'pre_get_posts', 'sort_by_custom_field' );

function sort_by_custom_field( $query ){
	// Make sure we're on the front end, that we're dealing with the main query and that we're on the post type archive for the product custom post type. Also that at least a year is set.
	if ( is_admin() || ! $query->is_main_query() || ! $query->is_post_type_archive( 'product' ) || ! $query->is_date() ) {
		return $query;
	}

	// Could shrink this down a lot, but let's be readable.

	$defaultYear = 2014;

	if ( isset( $query['query']['year'] ) ) {
		$queryYear = $query['query']['year'];
	} else {
		$queryYear = $defaultYear;
	}

	$outputFormat = 'Ymd';

	// The DateTime object stuff here looks scary if you're not used to it, but it's really not too bad.

	if ( $query->is_year() ) {
		$dateObj = DateTime::createFromFormat( '!Y', $queryYear );
		$queryStart = $dateObj->adjust( 'First day of the year' )->format($outputFormat);
		$queryEnd = $dateObj->adjust( 'Last day of the year' )->format($outputFormat);
		$queryValue = array( $queryStart, $queryEnd );
		$queryCompare = 'BETWEEN';
	}

	// make sure it's a monthly query and that a month is set.
	if ( isset( $query['query']['monthnum'] ) ) {
		$queryMonth = $query['query']['monthnum'];
	} 

	if ( $query->is_month() && $queryMonth ) {
		$dateObj = DateTime::createFromFormat( '!Ym', $queryYear . $queryMonth );
		$queryStart = $dateObj->adjust( 'First day of the month' )->format($outputFormat);
		$queryEnd = $dateObj->adjust( 'Last day of the month' )->format($outputFormat);
		$queryValue = array( $queryStart, $queryEnd );
		$queryCompare = 'BETWEEN';
	}

	// make sure it's a day query and that a day is set.
	if ( $query->is_day() && isset( $query['query']['day'] ) ) {
		$dateObj = DateTime::createFromFormat( '!Ymd', $queryYear . $queryMonth . $query['query']['day'] );
		$queryValue = $dateObj->format($outputFormat);
		$queryCompare = '=';
	}

	// Sort by meta value: http://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters
	// 
	// Select items which have a meta_key of release_date, where it's between Jan 01 2014 and now. 
	// Times would have to be stored in the format YYYYMMDD for this method. 
	// 
	// Then order descending by release date and then ascending by title

	$query->set( 'orderby' , array( 'meta_value_num' => 'DESC', 'title' => 'ASC' ) );

	if ( ! isset( $queryCompare ) || ! isset( $queryValue ) ) {
		return $query
	}

	$query->set( 'meta_key' , 'release_date' );
	$query->set( 'meta_query' , array( 
		array(
				'key' => 'release_date',
				'value' => $queryValue, // this will either be the exact day or the range of dates applicable
				'type' => 'DATE',
				'compare' => $queryCompare, // This will either be BETWEEN or =
			),
		) 
	);

	return $query;
}

This is quite verbose for readability.