[tz] bug in export TZ where day light savings spans Dec 31st?

[david singleton]

When I use the example from the timezone man page to define,  and export
a local TZ variable:

  export  TZ="NZST-12.00:00NZDT-13:00:00,M10.1.0,M3.3.0"

And then set the date to December 31st at 23:59 I see the clock roll over to 00:00
normally.

But when I define a timezone using a positive offset I see time get
set backwards at December 31st.   I believe time should not get set back
on December 31st for any timezones whose day light savings span December 31st.

  export  TZ="NZST+12.00:00NZDT+13:00:00,M10.1.0,M3.3.0"

  date  123123592011.55
Sat Dec 31 23:59:55  2011

  when I wait 5 seconds and check the date again I see the date has been
adjusted to Dec 31st at 12:00?

  root at dstest2:~# date
  Sat Dec 31 12:00:01 NZST 2011

What is happening with TZs defined with a positive offset?

I can reproduce this on Linux systems using glibc 2.5.90 up through glibc 2.12.1.

I was hoping you could help me understand what is going on?

thank you

Dave