[formerly_proven]

This does two things:

1.) Add an alternate way of specifying the time zone, which is possibly inconsistent with the time zone already specified in the time stamp, so there are multiple ways this might or might not be handled. Roughly 1/3 of the RFC seems to deal with that.

As the RFC tries to avoid saying: Do not use RFC9557/IXDTFs under any circumstance for any kind of information which will be used for security purposes; implementations will disagree what time stamp an IXDTF represents.

2.) Saying which calendar the time stamp actually refers to.

This does not provide a way to fix the "I'm actually referring to a fixed calendaric event and if the time zone changes, I don't want that event to shift" problem.