According to the documentation, time.strftime will use
time.localtime, when no time tuple is provided as
argument. So, I wonder if it is desired behavior that
%z returns different values in the following two cases:

Python 2.4.3 (#2, Apr 27 2006, 14:43:58)
[GCC 4.0.3 (Ubuntu 4.0.3-1ubuntu5)] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>> import time
>>> time.strftime('%Y-%m-%dT%H:%M:%S%z')
'2006-09-18T16:12:05+0200'
>>> time.strftime('%Y-%m-%dT%H:%M:%S%z', time.localtime())
'2006-09-18T16:12:05+0000'

The first behavior is what I am looking for.

I realize that %z is not documented, so maybe it should
be rejected instead of giving surprising results, like
above.

This behavior is observed on different Linux systems
under different versions of Python, e.g. on Ubuntu
Dapper Drake running Python 2.4.3.