Loop through existing posts and add year and month terms based on publish date

functions.php

/* ONLY RUN THIS FUNCTION ONE TIME */
function dp_set_publish_date_taxonomy_all_posts() {
    $posts = get_posts(array(
        'post_type' => 'post',
        'posts_per_page' => -1,
    ));

	foreach ($posts as $post) {
		$post_id = $post->ID;
		
		// Get the post year
		$post_year = get_the_date('Y', $post_id);

		// Assign the post to the term in the "post_date" taxonomy with the post year
		$post_year_term_id = dp_loop_check_create_term( $post_year, $post_id);

		if($post_year_term_id) {
			// Get the post Month
			$post_month = get_the_date('F', $post_id);

			// Assign the post to the term in the "post_date" taxonomy with the post month
			dp_loop_check_create_term( $post_month, $post_id, $post_year_term_id, $post_year);
		}
	}
}

// RUN FUNCTION AFTER THEME SETUP TO AVOID WP TAXONOMY DOES NOT EXIST ERROR
add_action( 'init', 'dp_set_publish_date_taxonomy_all_posts', 100);

function dp_loop_check_create_term($value, $post_id, $parent_term_id = null, $post_year = null) {
	$term = term_exists( $value, 'post_date', $parent_term_id);
	if( $term ) {
		// Term exists
		$existing_term_array = wp_set_post_terms( $post_id, $term['term_id'], 'post_date', true );
		if(!empty($existing_term_array) && is_array($existing_term_array)) {
			return $existing_term_array[0];
		}
	} else {
		// Create new term
		$args = [];
		if($parent_term_id) {
			$args = array(
				'parent' => $parent_term_id,
				'slug' => $value.'-'.$post_year,
			);
		}
		$new_term = wp_insert_term($value, 'post_date', $args);
		if( $new_term && !is_wp_error($new_term)) {
			$new_term_array = wp_set_post_terms( $post_id, $new_term['term_id'], 'post_date', true );
			if(!empty($new_term_array) && is_array($new_term_array)) {
				return $new_term_array[0];
			}
		}
	}
}