(I'm somewhat away from all that currently - and not aware if newest
Python versions already solved but:)

* a "time.asctime_tz([tim])" or so should deliver a full OS-indep.
_world_ time string incl. numeric timezone info like 
"Sat Mar 21 10:33:36 2009 +0000"
It should accept a 10-tuple (like urlopen_file.info().getdate_tz('date'),
or a time.time() float time, 
or interpret a 9-tuple as GMTIME/UTC.

* strftime("%z") should be supported on all OS - _constant numeric_
format "+0000" 

* strftime("%C",[tim]) should be like asctime_tz.
  it should accept as 2nd parameter a 10-tuple alternatively, 
  or a time.time() universal float time, 
  or interpret the 9-tuple as LOCALTIME as it were.

test cases to add: 
* simple render a variation of constant tuples/float_times to constant
result strings.