> It also says that  if "the offset to local time is unknown, this can
> be
> represented with an offset of "-00:00"". So I don't think we can write
> "Z" or "+00:00" if we don't know the UTC offset.

>> but Atom
>> specification mentions that

> It says "an uppercase "Z" character MUST be present in the absence of
> a numeric time zone offset". Correct me if I'm wrong, but "-00:00" 
> *is* a numeric time zone offset. So it isn't absent.

As quoted earlier, -00:00 is used when time zone is unknown, and "the absence of a numeric time zone offset" or "the absence of offset" is only possible when this time zone is unknown.