Assign category from custom field on post creation and update

by

After getting year, you may check if category exists using

category_exists($cat_name);

if not found then create category using

 wp_create_category( $cat_name, $parent );

after that you may create post using

$my_post = array(
  'post_title'    => wp_strip_all_tags( $_POST['post_title'] ),
  'post_content'  => $_POST['post_content'],
  'post_status'   => 'publish',
  'post_author'   => 1,
  'post_category' => array( 8,39 )
);

// Insert the post into the database
wp_insert_post( $my_post );

You may also add category to a existing post using

wp_set_post_categories($post_id,$post_categories)

more information:
https://developer.wordpress.org/reference/functions/wp_insert_post/
https://codex.wordpress.org/Function_Reference/wp_create_category
https://developer.wordpress.org/reference/functions/category_exists/