How to second orderby in "pre_get_posts" by meta value or combine single date and time to timestamp

I’m outlining a solution for what you asked in your last comment, because this wouldn’t been fitting for the comment format.

Code:

function date_time_to_timestamp_meta( $post_id ) {

    // meta fields are saved in DATETIME, e.g. »2013-12-09 22:32:12«
    // here is only the date part relevant
    $datetime_date = get_post_meta( $post_id, '_start_date', true );
    // here is only the time part relevant
    $datetime_time = get_post_meta( $post_id, '_start_time', true );

    // we're extracting the relevant part with date()
    $extracted_date = date('Y-m-d', $datetime_date);
    $extracted_time = date('G:i:s', $datetime_time);

    // we're bringing back together the extracted relevant date and time
    $datetime_date_and_time = $extracted_date . ' ' . $extracted_time;

    // we're constructing the TIMESTAMP with strtotime()
    $date_time_timestamp = strtotime( $datetime_date_and_time );

    // we're updating/creating our TIMESTAMP meta field
    update_post_meta( $post_id, '_date_time_timestamp', $date_time_timestamp ); 

}   
add_action( 'save_post', 'date_time_to_timestamp_meta', 111 );

Note: exemplary and untested

As this is just a proof of concept above function for sure is missing some things you normally want to have, like conditional checks on post type and/or meta fields. Read into the according php functions, if you’re not sure how to use them. I assumed a high priority for this, not knowing how and when the API calls you’re talking about are happening, in short, you of course want to make sure this is happening after the two single date and time fields are available with the actual information you need.

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