Resource list has "Date this was last changed", Element definition has "The date (and optionally time) when the value set was published. The date must change if and when the business version changes and it must change if the status code changes. In addition, it should change when the substantive content of the value set changes. (e.g. the 'content logical definition')." With Comment: "Note that this is not the same as the resource last-modified-date, since the resource may be a secondary representation of the value set. Additional specific dates may be added as extensions or be found by consulting Provenances associated with past versions of the resource."

I would assume this is the datetime when this version was saved. What is it?