Haha, alles is gemakkelijk als je het weet... (:P)
The *.tpl files are parsed by the PHP scripts of phpBB. Every variable {VARIABLE} is replaced by whatever one of the phpBB scripts put inside this variable.
For your example, put something like this in includes/page_header.php (before $template->pparse("overall_header");)
Code:
$template->assign_vars(array("MY_DATE"=> date("l"). ', ' . date("F j, Y") . ' ' . date("g:iA T") . ' '));
and in the *.tpl put {MY_DATE} everywhere you want the date print.
Take a look at includes/page_header.php, it will probably clear things up a little.