I am trying to find the creation date of changes to milestones in rally, which seems possible because it is a column under a milestone's revision history, but creation date is not a child of revisions or revision history.
Is there an alternative object I should try querying for?
The Revision type does have a CreationDate field (it's inherited from its base type, WorkspaceDomainObject). So if you load the Revisions collection of the RevisionHistory associated with your milestone you should have it. They also are returned chronologically, so the first revision will be the most recent change.