カスタムフィールドで指定した時間にカテゴリー（or タグ or カスタム分類のターム）を切り替えるクラス。Advanced Custom Fieldsに依存。

Scheduled_Term.php

<?php

Class Scheduled_Term {

	public $taxonomy = "category";
	public $term_id = 1;
	public $term_slug;

	public $event_key = "scheduled_term";
	public $date_key = 'date';
	public $time_key = 'time';
	public $post_id;
	public $date_time_key = "";


	public function init() {
		add_action('edit_post', array(&$this, "scheduled_event_fire") );
		add_action( $this->event_key, array(&$this, "set_term") );
	}

	public function scheduled_event_fire( $post_id ) {
		$this->post_id = $post_id;
		add_action('shutdown', array(&$this, "scheduled_event") );
	}

	protected function get_acf_meta( $post_id, $key ) {
		$acf_field = get_field_object( $key );
		$acf_key = $acf_field["key"];

		if(isset( $_POST["fields"][$acf_key]) ) {
			return $_POST["fields"][$acf_key];
		}
		else {
			return get_post_meta( $post_id, $key, 1 );
		}
		return false;
	}

	public function scheduled_event() {
		$post_id = $this->post_id;
		$post = get_post( $this->post_id );

		if( $this->date_time_key ) {
			$datetime = $this->get_acf_meta( $this->post_id, $this->date_time_key );
		}else {
			$date = $this->get_acf_meta( $this->post_id, $this->date_key );
			$time = " ".$this->get_acf_meta( $this->post_id, $this->time_key );
			$datetime = $date.$time;
		}

		wp_clear_scheduled_hook( $this->event_key, array( $post->ID ) ); //イベントの初期化
		$timestamp = strtotime( $datetime );
		$gmt = $timestamp - 9*60*60;
		if( $gmt ) {
			wp_schedule_single_event( $gmt, $this->event_key, array( $post->ID ,$date.$time)); //イベントのセット
		}

	}

	public function set_term( $post_id, $str = "" ) {
		if( $this->term_slug ) {
			$term = get_term_by( "slug", $this->term_slug, $this->taxonomy );
			$this->term_id = $term->term_id;
			$this->term_id = intval( $this->term_id );
		}

		wp_set_object_terms( $post_id, $this->term_id, $this->taxonomy );

	}
}

Use.php

<?php

/*

Advanced Custom Fieldsで作ったフィールドを使うことが前提。
get_acf_metaがACFに依存しているので、それをオーバーライドすれば、ACF以外でも動作します。
*/


//インスタンス化
$scheduled_term = new Scheduled_Term;

//指定した時間にカテゴリーをopenに変更
$scheduled_term->term_slug = "open";

//指定した時間にカテゴリーIDで指定
$scheduled_term->term_id = "10";

//term_idよりslugの方が優先的に使われる
$scheduled_term->term_id = "10";
$scheduled_term->term_slug = "open";

//タクソノミーを指定
$scheduled_term->taxonomy = "status";
$scheduled_term->term_slug = "closed";

//カスタムフィールドのキーを変更
$scheduled_term->date_key = "date";
$scheduled_term->time_key = "time";

//date_time_keyを指定した場合、time_keyとdate_keyは無視される。
$scheduled_term->date_time_key = "datetime";

//複数のイベントを発生させるときはevent_keyを変更
$scheduled_term->event_key = "event_2"
$scheduled_term->init();