2000-01-01+13:00 and 1999-12-31−11:00 are e qual?

"Costello, Roger L." <[email protected]> Mon, 17 Sep 2012 19:31:29 +0000

Newsgroups	gmane.text.xml.schema.devel
Message-ID	<[email protected]>

Hi Folks,

In the XSD 1.1 data types specification it says this in the discussion of the "date" data type:

    Some date values with different time zone offsets 
    that were identical in the 1.0 version of this 
    specification, such as 2000-01-01+13:00 and 
    1999-12-31−11:00, are in this version of this 
    specification equal ...

That seems to be a mistake. 

How can adding 13 hours onto January 1, 2000 equal December 31, 1999 minus 11 hours?

/Roger