原因是当您只设置了一个没有日期的时间时，php函数strtotime() 默认情况下，将今天添加为日期，因此如果时间为；04:30“；，时间戳在过去。

我这样修复：

  $timestamp = strtotime( $cron_sync_time );
  if( $timestamp < time() ){ //if the time already past today
      $timestamp = $timestamp + 60 * 60 * 24; //add 1 day
  }
  wp_schedule_event( $timestamp, \'daily\', $hook);

Full code

function scheduled_task_activation(){
    $hook = \'my_hook\';
    $options_values = get_option( "option_name" );
    $is_cron_active = (!empty( $options_values[\'cron-sync-active\'] ) ) ? true : false;
    $cron_sync_time = (!empty( $options_values[\'cron-sync-time\'] ) ) ? $options_values[\'cron-sync-time\']: "00:00:00";
    if( !$is_cron_active ){
        if( wp_next_scheduled( $hook ) ){
            wp_clear_scheduled_hook( $hook );
        }
    }
    else if ( ! wp_next_scheduled( $hook ) || ( $cron_sync_time !== get_option( "cron_time_used") )  ) {
        if( $cron_sync_time !== get_option( "cron_time_used" ) )
            wp_clear_scheduled_hook( $hook ); //avoid dupplication
        $timestamp = strtotime( $cron_sync_time );
        if( $timestamp < time() ){
            $timestamp = $timestamp + 60 * 60 * 24;
        }
        wp_schedule_event( $timestamp, \'daily\', $hook);
        update_option( "cron_time_used", $cron_sync_time );
    }
}