Explicitly specify how the Z timezone is interpreted in DateTime #524
Description
The original RFC-3339 defined the Z timezone offset as equivalent to the +00:00 offset (zero or no offset) while -00:00 was used to represent an offset that was unknown. However, since -00:00 is not permitted in ISO-8601, in practice Z was often used to mean that the offset is unknown and -00:00 avoided altogether. This convention was somewhat recently codified in RFC-9557, that among other changes redefines Z to explicitly mean "the time in UTC is known, but the offset to local time is unknown".
I think it would be worthwhile to explicitly mention in the definition of the DDLm DateTime content type how the Z offset should be interpreted. A short sentence like "The Z timezone offset should be interpreted as defined in RFC-9557" should suffice. Alternatively, we may forbid timestamps with the Z and -00:00 timezone offsets altogether.