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<?php

function plugin_schedules( $schedules ) {
  if ( ! isset( $schedules['30sec'] ) ) {
    $schedules['30sec'] = array(
      'interval' => 30,
      'display' => __( 'Once every 30 seconds' )
    );
  }
  return $schedules;
}

add_filter( 'cron_schedules','plugin_schedules' );

if ( ! wp_next_scheduled( 'plugin_cron_hook' ) ) {
  wp_schedule_event( time(), '30sec', 'plugin_cron_hook' );
}

add_action( 'plugin_cron_hook', 'plugin_cron_task' );

function plugin_cron_task() {
  // Task
}