Time and time zone requirements vs SDMX-ML

[hoehrmann]

https://github.com/sdmx-twg/sdmx-json/blob/71fe5eaa9fcd29e3c15f2f0216a19b9b650b1dbd/structure-message/docs/1-sdmx-json-field-guide.md defines the $.meta.prepared property as »prepared - String. A timestamp indicating when the message was prepared. Values must follow the ISO 8601 syntax for combined dates and times, including time zone.«

The corresponding element in SDMX-ML is not required to have associated time and time zone information. In SDMX-JSON the property is required. So what, then, is a converter from SDMX-ML to SDMLX-JSON supposed to do? It cannot omit the property, it cannot claim that the message has been prepared "now" as it just converts the syntax, and it cannot pretend it was prepared at midnight UTC as that would falsify the message.

(Also note that the example given in this section actually omits the time zone and is thus invalid.)

I suspect the same issue applies to other properties like validFrom and validTo.

(In doubt, please handle this as a public review comment on SDMX 3.1 once the comment period begins.)