Create blog post from external source and set publication date

I don’t know why you are loading WordPress by including the core files, there should be something wrong at that point.

Anyway, to schedule an event, you can use wp_schedule_single_event. This function accepts 3 arguments:

wp_schedule_single_event( $timestamp, $hook, $args );

In your case, you can wrap wp_insert_post in a function, and then call it when the time arrives:

// Use this instead of wp_insert_post
wp_schedule_single_event( 'SET THE TIME HERE', 'schedule_my_post' );

// Add an action that runs the function
add_action( 'schedule_my_post','publish_my_post' );

// Now, do the actual publish
function publish_my_post($args){
    wp_insert_post($args);
}

Done.