Check you php version.
>
>  From calendar project page on drupal.org:
> *NOTE: I basically have no more time to support problems related to
> outdated versions of PHP. If you want dates to work correctly you need
> to use PHP 5.2 or higher. Expect this to become an absolute requirement
> in the near future. It is already a requirement for the D7 version.*
    

Except that in this case, calendar needs *exactly* PHP 5.2, but not
higher. The above mentioned error you get if you try to use PHP 5.3...

Jarry