> The point is that your implementation doesn't allow people to generate > 'Z'-ending timestamp if they need a subset of rfc3339. That is exactly right. Do you have any suggestion, how to do that? Maybe an optional argument to the rfcformat method? Something like: def new_rfcformat(self, default_utcoffset='-00:00'): if we_know_the_utc_offset: return self.rfcformat() # the method in my current patch else: return self.isoformat() + default_utcoffset This way, if somebody want 'Z' instead of '-00:00', just calls the method like this: d.new_rfcformat('Z') Though this way '+00:00' wouldn't get replaced by 'Z'. Any suggestions?