Creating a RFC 1123 compliant date for HTTP header creation on the CLI

env TZ=GMT; date '+%a, %d %b %Y %T %Z'

1. %T is equivalent to %H:%M:%S.
2. %Z is replaced by the time zone name.

The solution by @brad, DATE=$(date -u +%a,\ %d\ %b\ %Y\ %H:%M:%S\ GMT), would be incorrect if the time zone setting of your system is not GMT. For example:

env TZ=Asia/Taipei; date -u +%a,\ %d\ %b\ %Y\ %H:%M:%S\ GMT

%H:%M:%S would be GMT+8 while the output is GMT, but should be CST.