It was minuted in the meeting of 2021-06-09 that best practice for time values is to include a time zone qualifier. Interestingly, not also for date values, though ISO8601 has it available for date values.

This was raised when noting that JSON schema requires the time zone qualifier and an instance is considered not schema-valid with the qualifier absent.

[ https://json-schema.org/draft/2020-12/json-schema-validation.html#rfc.section.7.3.1|https://json-schema.org/draft/2020-12/json-schema-validation.html#rfc.section.7.3.1]

https://datatracker.ietf.org/doc/html/rfc3339#section-5.6

Agreed at the meeting that UBL 2.4 should document this best practice and that the XML instances included in the distribution should be modified such that every time value has a meaningful time zone qualifier.

https://lists.oasis-open.org/archives/ubl/202106/msg00010.html