rfc1036 December 1987

2.2.4.  Expires

    This line, if present, is in a legal USENET date format.  It
    specifies a suggested expiration date for the message.  If not
    present, the local default expiration date is used.  This field is
    intended to be used to clean up messages with a limited usefulness,
    or to keep important messages around for longer than usual.  For
    example, a message announcing an upcoming seminar could have an
    expiration date the day after the seminar, since the message is not
    useful after the seminar is over.  Since local hosts have local
    policies for expiration of news (depending on available disk space,
    for instance), users are discouraged from providing expiration dates
    for messages unless there is a natural expiration date associated
    with the topic.  System software should almost never provide a
    default "Expires" line.  Leave it out and allow local policies to be
    used unless there is a good reason not to.