Changeing the expiry date of a sub key

[john s.]

Please help me to understand what is going on here when I attempt to change the
expiry date of a sub key. This is a "learning by doing" key.  

john at erica:~/Desktop$ gpg -K
/home/john/.gnupg/secring.gpg
-----------------------------
sec   3072R/2419BD42 2014-06-15 [expires: 2014-06-21]
uid                  John R Song <john at ageinggracefully.ca>
uid                  [jpeg image of size 6326]
ssb   3072R/C6A5A9DB 2014-06-15

john at erica:~/Desktop$ gpg -k
/home/john/.gnupg/pubring.gpg
-----------------------------
pub   3072R/2419BD42 2014-06-15 [expires: 2014-06-21]
uid                  John R Song <john at ageinggracefully.ca>
uid                  [jpeg image of size 6326]
sub   3072R/C6A5A9DB 2014-06-15 [expires: 2015-06-15]

john at erica:~/Desktop$ gpg --edit-key C6A5A9DB
gpg (GnuPG) 1.4.16; Copyright (C) 2013 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  3072R/2419BD42  created: 2014-06-15  expires: 2014-06-21  usage: SC  
                     trust: ultimate      validity: ultimate
sub  3072R/C6A5A9DB  created: 2014-06-15  expires: 2015-06-15  usage: E   
[ultimate] (1). John R Song <john at ageinggracefully.ca>
[ultimate] (2)  [jpeg image of size 6326]

gpg> expire C6A5A9DB
Changing expiration time for the primary key.
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 5
Key expires at Sat 21 Jun 2014 07:47:35 PM EDT
Is this correct? (y/N) y

pub  3072R/2419BD42  created: 2014-06-15  expires: 2014-06-21  usage: SC  
                     trust: ultimate      validity: ultimate
sub  3072R/C6A5A9DB  created: 2014-06-15  expires: 2015-06-15  usage: E   
[ultimate] (1). John R Song <john at ageinggracefully.ca>
[ultimate] (2)  [jpeg image of size 6326]

gpg> save

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