nt:file and nt:folder both extend a common supertype, nt:hierarchyNode. nt:hierarchyNode defines one property, a protected, mandatory DATE property named jcr:created that is supposed to be automatically set with the create time of the node.
We've been relying on the underlying connector to populate this field, which works great when the underlying connector is a file system connector or an SVN connector. However, other connectors (e.g., the IMR connector or Infinispan connector do not populate this field).
I think the correct fix for this is out in the JCR layer anyway. This is where we handle other protected properties and it would keep us from having to recode solutions for this in every connector.