function jetpackcom_custom_sync_schedule( $schedules ) {
    if ( ! isset( $schedules['30min'] ) ) {
        $schedules['30min'] = array(
            'interval' => 30 * MINUTE_IN_SECONDS,
            'display' => __( 'Every 30 minutes' ),
        );
    }
    return $schedules;
}
add_filter( 'cron_schedules', 'jetpackcom_custom_sync_schedule' );
 
function jetpackcom_return_30_min() {
    return '30min';
}
add_filter( 'jetpack_sync_incremental_sync_interval', 'jetpackcom_return_30_min' );
add_filter( 'jetpack_sync_full_sync_interval', 'jetpackcom_return_30_min' );