Hi,
I have a case where I have to order posts by date and post name.

array[3] {
  [0] {
    ["post_date"]=> string(19) "2016-10-10 18:51:08",
    ["post_name"]=> string(26) "aaa"
  },
  [1] {
    ["post_date"]=> string(19) "2016-12-11 16:51:08",
    ["post_name"]=> string(26) "ccc"
  },
  [2] {
    ["post_date"]=> string(19) "2016-10-10 17:51:08",
    ["post_name"]=> string(26) "bbb"
  }
}

Expected result:

array[3] {
  [0] {
    ["post_date"]=> string(19) "2016-12-11 16:51:08",
    ["post_name"]=> string(26) "ccc"
  },
  [1] {
    ["post_date"]=> string(19) "2016-10-10 18:51:08",
    ["post_name"]=> string(26) "aaa"
  },
  [2] {
    ["post_date"]=> string(19) "2016-10-10 17:51:08",
    ["post_name"]=> string(26) "bbb"
  }
}

The issue of ordering by date is that post’s date is in a datetime format (Y-m-d hh:mm:ss) and is usually unique, so the ‘Secondary Sort Order’ option is useless. Is it possible to sort posts by date (Y-m-d) instead.

I found a solution for initial form setup and display, but cannot alter the ajax calls:

<?php
function query_orderby_day( $orderby ) {
	global $wpdb;
	return str_replace( "{$wpdb->posts}.post_date", "CAST({$wpdb->posts}.post_date AS DATE)", $orderby );
	}

	// Add the filter to change the ORDER clause
	add_filter( 'posts_orderby', 'query_orderby_day', 10, 2 );

	the_content();

	// Remove the filter
	remove_filter( 'posts_orderby', 'query_orderby_day' );
?>