Yesterday we talked about defining a datetime field for the table created by custom module.
Drupal 7 – Create a Datetime field in hook_schema() @ 1

As i have mentioned in the above post, the datetime field is no longer support by the Drupal Schema API. If you don’t want to use the datetime field, you can try to save the datetime in timestamp format and stored it in an int column.

<?php

function <module>_schema() {
  $schema = array();
  
  $schema['<table name>'] = array(
    'description' => t('<table description>'),
    'fields' => array(
      'id' => array(
        'description' => t('The primary identifier.'),
        'type' => 'serial',
        'unsigned' => TRUE,
        'not null' => TRUE
      ),
      'created_at' => array(
        'description' => t('The creation date timestamp.'),
        'type' => 'int',
        'unsigned' => TRUE,
        'not null' => TRUE
      ),
    ),
    'primary key' => array('id'),
  );
  
  return $schema;
}

You can save the current timestamp as follow.

$id = db_insert('<table name>')
  ->fields(array(
    'created_at' => REQUEST_TIME
  )
)
->execute();

Done =)

Reference:

• Schema API – Data types
• StackOverflow – drupal 7 custom schema error datetime