Why is "Expires" 1981?
Solution 1:
It's an attempt to disable caching.
The date is the birthday of the developer Sascha Schumann who added the code.
From session.c:
Authors: Sascha Schumann <[email protected]>
Andrei Zmievski <[email protected]>
// ...
CACHE_LIMITER_FUNC(private)
{
ADD_HEADER("Expires: Thu, 19 Nov 1981 08:52:00 GMT");
CACHE_LIMITER(private_no_expire)(TSRMLS_C);
}
Solution 2:
HTTP Expires header
http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
It is set to negative or past value, to prevent caching of response.
Quite common usage of this header.