WordPress function to create or update custom taxonomy terms from an ACF date-picker on post save.

manga-release.php

<?php
function create_manga_categories( $post_id ) {
	$date = get_field('manga_group')['manga_release_date'];
	if(get_post_type( $post_id ) == 'manga' && $date) {
		$date_convert = DateTime::createFromFormat('Ymd', $date);
		$date_converted = $date_convert->format('F Y');
		$date_converted2 = $date_convert->format('F j');
		$date_converted3 = $date_convert->format('F-j-Y');
		$terms = [];
		$parent_term_check = get_term_by('name', $date_converted, 'manga_release_date', 'ARRAY_A');
		if(!$parent_term_check) {
			$parent_term = wp_insert_term($date_converted, 'manga_release_date');
			$parent_term_id = $parent_term['term_id'];
		} else {
			$parent_term_id = $parent_term_check['term_id'];
		}
		$terms[] = $parent_term_id;
		$child_term_check = get_term_by('slug', $date_converted3, 'manga_release_date', 'ARRAY_A');
		if(!$child_term_check) {
			$child_term = wp_insert_term($date_converted2, 'manga_release_date', array('parent'=>$parent_term_id, 'slug'=>$date_converted3));
			$terms[] = $child_term['term_id'];
		} else {
			$terms[] = $child_term_check['term_id'];
		}
		wp_set_object_terms( $post_id, $terms, 'manga_release_date' );
	}
	elseif( get_post_type( $post_id) == 'manga' ) {
		wp_set_object_terms( $post_id, null, 'manga_release_date');
	}
}
add_action('acf/save_post', 'create_manga_categories', 20);