When a node is changed to add the 'mix:created' mixin (or created with a primary node type that extends that mixin), the 'jcr:created' and 'jcr:createdBy' properties defined by that mixin should be automatically set. See Section 3.7.11.7 of the JCR 2.0 specification.

Similarly, when a node is modified 'significantly' (e.g., added/moved/removed children, added/changed/removed properties) and it is of the 'mix:lastModified' mixin, it's 'jcr:lastModified' and 'jcr:lastModifiedBy' properties need to be changed. See section 3.7.11.8 of the spec.