Why is the Pressflow Last-Modified header not like plain Drupal? The Last-Modified header is set in includes/bootstrap.inc. Plain Drupal uses 

header("Last-Modified: ". gmdate("D, d M Y H:i:s") ." GMT");


which yields 

Last-Modified: Sun, 27 Jan 2013 21:52:57 GMT


Pressflow uses

$default_headers = array(
    'Last-Modified' =&gt; gmdate(DATE_RFC1123, $_SERVER['REQUEST_TIME']),


which yields

Last-Modified: Sun, 27 Jan 2013 21:52:57 +0000


'+0000' is incorrect according to RFC 2616, and is flagged as an invalid date by http://redbot.org/ 

What gives? Is this a Pressflow performance technique, or an error, or what? Do browsers care? 

We're trying to nail down some load issues on our site, and want to be sure we are not getting more repeat requests than is appropriate.
Drupal 6 sets the Last-Modified header with

header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");


That was changed to

$default_headers = array(
  'Expires' =&gt; 'Sun, 19 Nov 1978 05:00:00 GMT',
  'Last-Modified' =&gt; gmdate(DATE_RFC1123, REQUEST_TIME),


during the development of drupal7. Pressflow has backported the patch from drupal7 in 2009. That's the reason of the difference. As for the possible impact of this, in fact it can have bad effects on caching. At least squid 3.1.10 prints a warning about the header and refuses to store the page in cache

cannot parse hdr field: 'Last-Modified: Tue, 12 Feb 2013 14:35:43 +0000'
[snip]...[/snip]
refreshIsCachable() returned non-cacheable..


Switching to the "GMT" version resolves the issue with squid.

rfc2616 (HTTP/1.1: Protocol Parameters:) specifies that "Last-Modified" header value can conform to rfc1123 (which looks OK with +0000) but all the examples use "GMT".

I think it'll be better to create a bug report in drupal issue tracker and see what the developers answer.