diff --git a/bookkeeping.cabal b/bookkeeping.cabal
--- a/bookkeeping.cabal
+++ b/bookkeeping.cabal
@@ -1,5 +1,5 @@
 name:                bookkeeping
-version:             0.3.2.0
+version:             0.3.3.0
 synopsis:            A module for bookkeeping by double entry.
 description:
     A module for bookkeeping by double entry. This module provides a way to do bookkeeping programmatically.
diff --git a/src/Business/Bookkeeping.hs b/src/Business/Bookkeeping.hs
--- a/src/Business/Bookkeeping.hs
+++ b/src/Business/Bookkeeping.hs
@@ -221,15 +221,15 @@
 
 newtype Year = Year
   { unYear :: Integer
-  } deriving (Show, Read, Ord, Eq, Num, Enum)
+  } deriving (Show, Read, Ord, Eq, Num, Enum, Real, Integral)
 
 newtype Month = Month
   { unMonth :: Int
-  } deriving (Show, Read, Ord, Eq, Num, Enum)
+  } deriving (Show, Read, Ord, Eq, Num, Enum, Real, Integral)
 
 newtype Date = Date
   { unDate :: Int
-  } deriving (Show, Read, Ord, Eq, Num, Enum)
+  } deriving (Show, Read, Ord, Eq, Num, Enum, Real, Integral)
 
 newtype Description = Description
   { unDescription :: Text
@@ -247,7 +247,7 @@
 
 newtype Amount = Amount
   { unAmount :: Int
-  } deriving (Show, Read, Ord, Eq, Num)
+  } deriving (Show, Read, Ord, Eq, Num, Enum, Real, Integral)
 
 newtype DebitCategory = DebitCategory
   { unDebitCategory :: Category