Drupal 7 function db_insert() causes internal server error (500) - mysql

Question

I am building an onsite payment method and among other things I need to save data to a custom db table, after recieving a callback from an external API service.
This is the function:
// callback from external service acting as client
function _api_callback() {
global $user;
$data = json_decode(file_get_contents("php://input"), $assoc = true);
$date = $date['year'] . '-' . $date['month'] . '-' . $date['day'];
$timestamp = strtotime($date);
db_insert('postback')
->fields(array(
'user_id' => $user->uid,
'data' => $data,
'created_date' => $timestamp,
))
->execute(); // save data from external service
}
In the site's access log I can see that my site responds with a 500 code when the external callback arrives.
However if I comment out everything in that function the external callback recieves a 200 response code.
So there is some thing going wrong, when this callback is recieved. It's hard to debug since the callback from the external API service starts a whole new session.
The custom table "postback" was successfully created in the .install file of my custom module and I have checked in phpMyAdmin that the table is present and well. This is the schema() function in the .install file:
function module_payments_schema() {
$schema['postback'] = array(
'description' => 'Data saved from API postback URL.',
'fields' => array(
'id' => array(
'description' => 'Auto incremental id.',
'type' => 'serial',
'unsigned' => TRUE,
'not null' => TRUE),
'user_id' => array(
'description' => 'User id for the order.',
'type' => 'int',
'unsigned' => TRUE,
'not null' => FALSE),
'order_id' => array(
'description' => 'Current order number.',
'type' => 'int',
'unsigned' => TRUE,
'not null' => TRUE,
'default' => 0),
'data' => array(
'description' => 'Postback data from external API.',
'type' => 'varchar',
'length' => 1020,
'not null' => TRUE,
'default' => ''),
'created_date' => array(
'description' => 'Created date and time (yyyy-mm-dd H:i:s).',
'type' => 'varchar',
'mysql_type' => 'DATETIME',
'not null' => TRUE,
),
),
'primary key' => array('id'),
);
return $schema;
}
Anyone who can see what's wrong?

Answer 1

It turns out I needed to format the date value like this...
date('Y-m-d H:i:s');
... in order to work with the mysql DATETIME type.
So the working function now looks like this:
function _api_callback() {
global $user;
$data = json_decode(file_get_contents("php://input"), $assoc = true);
db_insert('postback')
->fields(array(
'user_id' => $user->uid,
'data' => $data,
'created_date' => date('Y-m-d H:i:s');
))
->execute(); // save data from external service
}