rfc1036 December 1987

2.1.2.  Date

    The "Date" line (formerly "Posted") is the date that the message was
    originally posted to the network.  Its format must be acceptable
    both in RFC-822 and to the getdate(3) routine that is provided with
    the Usenet software.  This date remains unchanged as the message is
    propagated throughout the network.  One format that is acceptable to
    both is:

            Wdy, DD Mon YY HH:MM:SS TIMEZONE

    Several examples of valid dates appear in the sample message above.
    Note in particular that ctime(3) format:

                Wdy Mon DD HH:MM:SS YYYY

    is not acceptable because it is not a valid RFC-822 date.  However,
    since older software still generates this format, news
    implementations are encouraged to accept this format and translate
    it into an acceptable format.

    There is no hope of having a complete list of timezones.  Universal
    Time (GMT), the North American timezones (PST, PDT, MST, MDT, CST,
    CDT, EST, EDT) and the +/-hhmm offset specifed in RFC-822 should be
    supported.  It is recommended that times in message headers be
    transmitted in GMT and displayed in the local time zone.