Drupal fun fact :)
Interesting code in Drupal 7 core.
// HTTP/1.0 proxies does not support the Vary header, so prevent any caching
// by sending an Expires date in the past. HTTP/1.1 clients ignores the
// Expires header if a Cache-Control: max-age= directive is specified (see RFC
// 2616, section 14.9.3).
$default_headers['Expires'] = 'Sun, 19 Nov 1978 05:00:00 GMT';
The code is okay.
But I was wondering about the date. Why its 11/19/1978.. why not 1970??
Then I found this: http://en.wikipedia.org/wiki/Dries_Buytaert - read the first line. It's Dries's (Drupal Creator) birthday.
Poor Dries! I've set a reminder on 11/19 to wish you happy birthday!