My .htaccess file:

Header add X-Hello "time %D"

Now I determine what it really appears like, browse the X-Hello header:

$curl -v http://foo.com/bar.php
...
< HTTP/1.1 200 OK
< Date: Wed, 06 Oct 2010 20:43:39 GMT
< Server: Apache
< Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Pragma: no-cache
< Set-Cookie: PHPSESSID=7d32d8fch8qrnnuvoqqopr74c0; path=/
< X-Hello: time %D
< Transfer-Encoding: chunked
< Content-Type: text/html

That is clearly unlike the Apache documentation below. Can there be another directive that mod_headers needs to be able to interpolate atmosphere variables?

http://httpd.apache.org/docs/2.0/mod/mod_headers.html#examples

Without note otherwise, host company accepted to still using Apache 1.3 which doesn't interpolate values in mod_header.