Get next/previous post by entry date

helpers.php

<?php
/**
 * Get next/preview post by post_date
 * 
 * @param int|WP_Post 	$post
 * @param boolean 	$next_post
 * @return WP_Post|WP_Error
 */
function get_next_prev_post_by_date( $post, $next_post = true ) 
{
	global $wpdb;

	// get current post data
	if ( !is_a( $post, 'WP_Post' ) )
		$post = get_post( $post );

	// build select SQL query
	$query = "SELECT * FROM {$wpdb->posts}";
	$query .= " WHERE post_date ". ( $next_post ? '>' : '<' ) ." %s";
	$query .= " AND ID NOT IN (%d)";
	$query .= " ORDER BY post_date DESC LIMIT 1";

	// run query
	$target_post_row = $wpdb->get_row( $wpdb->prepare( $query, $post->ID, $post->post_date ) );

	// return post object if found
	if ( $target_post_row )
		return new WP_Post( $target_post_row );

	// return error if no match found
	return new WP_Error( 'no_match_found', __( 'No matched post found' ) );
}