ThreeTen/threeten — pull request #150 by RogerRiggs (2012-11-15 23:27:19+00:00)
Title: Issue 145 Chrono inheritence with (adjuster) method

Take a look and see if this is what you had in mind.
Casting to the implementation class may more effective, even for private date classes.
Let me know if I should amend this.