A 500 errors probably means your server it not liking the .htaccess file. It might be set up to not allow you to override the settings in this way and that is why it is producing a 500 error. That means the server didn't like something and since that is the only new setting, it must be that.
The admin can change this setting easily on their end or allow you to selectively overwrite it. They should have the setting anyway since anyone who tries to use the date function will encounter the same error.
The only other thing you could try is adding this code to the top of the php page giving you the problem. It basically just hides errors from displaying - but I don't know if that will really fix the problem or just mask it.
<?php ini_set('display_errors', 'off'); ?>