iRule to display one time warning message

I just wanted to comment that this doesn't work, once the cookie expires, the cookie isn't reset:

when HTTP_REQUEST {
    if { not ( [HTTP::cookie exists BANNERMESSAGE] ) } {
        set expires [clock format [clock seconds] -format "%A, %d-%b-%Y 20:00:00"]
        HTTP::respond 200 content "html banner content" "Set-Cookie" "BANNERMESSAGE=1; path=/; expires=$expires"
    }
}

This does though:

when HTTP_REQUEST {
    if { not ( [HTTP::cookie exists BANNERMESSAGE] ) } {
        set expires "28800"
        HTTP::respond 200 content "html banner content" "Set-Cookie" "BANNERMESSAGE=1; path=/; Max-Age=$expires"
    }
}