xml schema Timezone

I think that your interpretation might be off; I can't see how you ended up with GMT+13:00 from 13:00:00Z.
The XSD spec gives the following example:

2002-10-10T12:00:00+05:00 is 2002-10-10T07:00:00Z 
2002-10-10T00:00:00+05:00 is 2002-10-09T19:00:00Z

A nonnegative duration means that the time zone is ahead; negative is behind.
Assuming that the timestamp was taken at midnight (12:00AM), and it matched 13:00:00Z, then you could offset this either ahead or behind:

Behind: 2012-02-04T00:00:00-13:00 is 2012-02-04T13:00:00Z
Ahead:  2012-02-05T00:00:00+11:00 is 2012-02-04T13:00:00Z

The only one valid is the ahead (there is no -13); As to what is in that time zone, take a look here.

http://stackoverflow.com/questions/9950406/xml-schema-timezone