Use aeson's Day instance

servant-examples/servant-examples.cabal

@@ -21,7 +21,7 @@ executable tutorial
   main-is: tutorial.hs
   other-modules: T1, T2, T3, T4, T5, T6, T7, T8, T9, T10
   build-depends:
-      aeson >= 0.8
+      aeson >= 0.10
     , base >= 4.7 && < 5
     , bytestring
     , directory

servant-examples/tutorial/T2.hs

@@ -17,11 +17,6 @@ data User = User
   , registration_date :: Day
   } deriving (Eq, Show, Generic)
 
--- orphan ToJSON instance for Day. necessary to derive one for User
-instance ToJSON Day where
-  -- display a day in YYYY-mm-dd format
-  toJSON d = toJSON (showGregorian d)
-
 instance ToJSON User
 
 type UserAPI = "users" :> Get '[JSON] [User]