How set a timezone in information-bar?

Hello

I try setup a timezone [ my site is in GMT - 0300] in information-bar.php
I see the code is :

<br />
	<div id="information-bar"><br />
		<div class="alignright"><?php echo stripslashes($site_message); ?></div><br />
			<?php serverdate(); ?><br />
	</div><br />

But I don't find this function in php manual.
Well I need GMT -0300 [America/Brasil/Rio de Janeiro]

How I can adjust this?
Thank you
joao barroca