warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'onthisdate_

Here is the full warning:

warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'onthisdate_admin' was given in C:\wamp\www\drupal\includes\form.inc on line 371.

I have created a menu item like that:
$items['admin/settings/onthisdate'] = array(
'title' => 'On this date module settings',
'description' => 'Description of your On this date settings page',
'page callback' => 'drupal_get_form',
'page arguments' => array('onthisdate_admin'),
'access arguments' => array('access administration pages'),
'type' => MENU_NORMAL_ITEM,
);

When I click on the link "On this date module settings". It show me the given warning.

anybody know the reason of this warning?
Thanks.