Last-Modified doesn't even have to be a valid date string: http://www.nikcub.com/posts/persistant-and-unblockable-cookies-using-http-headers http://www.nikcub.

Last-Modified doesn't even have to be a valid date string: http://www.nikcub.com/posts/persistant-and-unblockable-cookies-using-http-headers http://www.nikcub.com/posts/persistant-and-unblockable-cooki... As an update to that post from 2011, I never did get the browsers to update their parsers. This E-Tag bug has been known for over a decade. As part of my post I tried to find the earliest written record of it and found a post in 2003: http://www.arctic.org/~dean/tracking-without-cookies.html http://www.arctic.org/~dean/tracking-without-cookies.html It seems to get 'rediscovered' at least a couple of times a year.