Admin Keepalive timer

[alhakeem2001]

Guys I've installed the Admin keepalive timer on my zc 1.5.1, Template template333 , But once I get warned that my time has been expired ( If I couldn't get the first warning-in case if I was away of computer ! ), I had to log in again, so once I fill-in my admin and pass words to go into my admin panel I get a blank page and if I remove in the URL at the top of my brawser all the text after my admin's name, I could get into my admin panel again and so far I could go every where in there except I couldn't add a new product as I'm getting the blank page again once I click on add a new product ! the log file is pointing to this line in that page > admin/keepalive.php on line 12
Which is: header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
and the whole log's info is:

PHP Warning: Cannot modify header information - headers already sent by (output started at /home1/public_html/newadmin/includes/keepalive_module.php:34) in /home1/public_html/newadmin/keepalive.php on line 12


Is that happening because I changed the time format from the US ( M,D,Y) to GB ( D,M,Y)? and how to fix the problem with the blank page ?