The fact that ISO8601 does not store time zones (only fixed UTC offsets, which is not the same thing) obviously has something to do with ISO8601. I'm not sure what you're going on about?
I’m saying the problem is the time zone, not the fixed offset. The fixed offset always means a specific time (which may or may not be the time in a particular place, due to problems with the definition of time zones). Times for dates in the future are a problem due to time zones, not due to offsets. If you know the offset, the time is exactly specified.