Complex WP_Query order request: DESC by day, but then ASC by time

Question

I've got a site with a couple thousand time-stamped posts in a custom post type. These get displayed by a few taxonomies and a date query in various views (I've got that all working just fine already). The tricky part is ordering them by a unique date format: the client insists that posts should be ordered DESC by day, then ASC by time within each day. As far as I can see in the Codex, order only takes date as a param, not day & time as distinguishable values. Is there any way to accomplish what I'm looking for without substantial re-writes of a half-dozen archive-tax template files?

I'm using the following to sort DESC by date already and would love a solution that fits into this function:

function my_post_sort( $vars ) {
    if ( is_tax(array('series', 'speakers', 'topics', 'venues'))) {
        $vars['order'] = 'ASC';
    }
    return $vars;
}
add_filter('query_vars', 'my_post_sort');

Answer 1

You're not using query_vars quite right - this filter is for whitelisting "public" query vars, not actually setting their values. It's also called rather early (wp::parse_request), so any conditional tags like is_tax() will always be false at this stage.

Use the pre_get_posts action for overriding the default query. Having said that, you need a custom ORDER BY clause. Use the posts_orderby filter:

function wpse_179686_posts_orderby( $orderby, $wp_query ) {
    if ( $wp_query->is_main_query() && $wp_query->is_tax( array( 'series', 'speakers', 'topics', 'venues' ) ) )
        $orderby = 'DATE( post_date ) DESC, TIME( post_date ) ASC';

    return $orderby;
}

add_filter( 'posts_orderby', 'wpse_179686_posts_orderby', 10, 2 );