The timestamp “DOI last updated” that you see on the left of the DOI record in Fabrica indicates the last time the DOI was touched by something.
As well as updates to the DOI record by a user, the timestamp also changes following internal updates from DataCite. Re-indexing is the most typical reason. However, there are many reasons for this timestamp to change and does not indicate that the metadata was updated. This feature is useful for things like caching.
The DOI created date is the date the DOI is registered in the DataCite system. The DOI registered date refers to when it was "registered" in the global handle server. If using a "draft" state workflow, the created and registered date and time could be different.
Updated 4 days ago