Unable calculate date field using strtotime
I got it. It should be:

// Extract the first field item
$date_items = field_get_items('node', $node, 'field_date_start_rent');
$date_item = array_shift($date_items);
$integer_items = field_get_items('node', $node, 'field_subscription_period');
$integer_item = array_shift($integer_items);

// Get the field value
$start_rent = $date_item['value'];
$period = $integer_item['value'];

// Update the other field
$node-&gt;field_cubagan[$node-&gt;language][0]['value'] = date('Y-m-d h:i:s', strtotime("+$period year",strtotime($start_rent)));

// Save the node
node_save($node);