eZ publish will generate invalid HTTP dates on Expires and Last-Modified headers, the problem is the missing " GMT" at the end of the date.
These headers must have dates conforming to the RFC1123 standard:
http://www.freesoft.org/CIE/RFC/1945/14.htm

This has been fixed in some places by earlier bugfixes but there still are some errors present.
They can be found with:

grep -H -n -i gmdate -R kernel lib

or

grep -H -n -i -E 'expires|last-modified' -R kernel lib