iRule adding extra time to HTTP::cookie expires <cookiename> 1200 relative command

it seems okay here. i am using 11.5.1. sgt is gmt+8.

// HTTP::cookie expires test 1 relative

 config

root@(B6900-R69-S40)(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm virtual bar
ltm virtual bar {
    destination 100.100.100.41:80
    ip-protocol tcp
    mask 255.255.255.255
    pool foo
    profiles {
        http { }
        tcp { }
    }
    rules {
        qux
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
    vs-index 2
}
root@(B6900-R69-S40)(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm rule qux
ltm rule qux {
    when HTTP_RESPONSE {
  HTTP::cookie insert name test value 1234 path / version 0
  HTTP::cookie expires test 1 relative
}
}

 test

[root@client3 ~] date; curl -I http://100.100.100.41
Tue Aug  5 15:41:01 SGT 2014
HTTP/1.1 200 OK
Date: Tue, 05 Aug 2014 07:28:45 GMT
Server: Apache/2.2.3 (CentOS)
Last-Modified: Sun, 09 Feb 2014 08:39:51 GMT
ETag: "41879c-59-2a9c23c0"
Accept-Ranges: bytes
Content-Length: 89
Content-Type: text/html; charset=UTF-8
Set-Cookie: test=1234;expires=Tue, 05-Aug-2014 07:41:03 GMT;path=/;

// HTTP::cookie expires test 1300 relative

 config

root@(B6900-R69-S40)(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm rule qux
ltm rule qux {
    when HTTP_RESPONSE {
  HTTP::cookie insert name test value 1234 path / version 0
  HTTP::cookie expires test 1300 relative
}
}

 test

[root@client3 ~] date; curl -I http://100.100.100.41
Tue Aug  5 15:42:29 SGT 2014
HTTP/1.1 200 OK
Date: Tue, 05 Aug 2014 07:30:13 GMT
Server: Apache/2.2.3 (CentOS)
Last-Modified: Sun, 09 Feb 2014 08:39:51 GMT
ETag: "41879c-59-2a9c23c0"
Accept-Ranges: bytes
Content-Length: 89
Content-Type: text/html; charset=UTF-8
Set-Cookie: test=1234;expires=Tue, 05-Aug-2014 08:04:10 GMT;path=/;