Create an acf/save_post filter that runs after acf http://www.advancedcustomfields.com/resources/acfsave_post/
Be sure to check the post type of the post just saved to prevent an infinite loop.
add_action('acf/save_post', 'create_payslip');
function create_payslip($post_id) {
if (get_post_type($post_id) != 'timesheet') {
return;
}
// get values from $post_id
// use wp_insert_post() to create new post
// use update_field() to update any acf fields
}