diff --git a/Utility/QuickCheck.hs b/Utility/QuickCheck.hs
new file mode 100644
index 0000000000..e13b6c00d9
--- /dev/null
+++ b/Utility/QuickCheck.hs
@@ -0,0 +1,37 @@
+{- QuickCheck instances
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# LANGUAGE TypeSynonymInstances #-}
+
+module Utility.QuickCheck where
+
+import Test.QuickCheck
+import Data.Time.Clock.POSIX
+import System.Posix.Types
+
+{- Times before the epoch are excluded. -}
+instance Arbitrary POSIXTime where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+instance Arbitrary EpochTime where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+{- Pids are never negative, or 0. -}
+instance Arbitrary ProcessID where
+	arbitrary = arbitrarySizedBoundedIntegral `suchThat` (> 0)
+
+{- Inodes are never negative. -}
+instance Arbitrary FileID where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+{- File sizes are never negative. -}
+instance Arbitrary FileOffset where
+	arbitrary = nonNegative arbitrarySizedIntegral
+
+nonNegative :: (Num a, Ord a) => Gen a -> Gen a
+nonNegative g = g `suchThat` (>= 0)
diff --git a/test.hs b/test.hs
index efd264cc80..77cb0cd409 100644
--- a/test.hs
+++ b/test.hs
@@ -6,7 +6,6 @@
  -}
 
 {-# OPTIONS_GHC -fno-warn-orphans #-}
-{-# LANGUAGE TypeSynonymInstances #-}
 
 import Test.HUnit
 import Test.HUnit.Tools
@@ -21,6 +20,7 @@ import System.IO.HVFS (SystemFS(..))
 import Text.JSON
 
 import Common
+import Utility.QuickCheck ()
 
 import qualified Utility.SafeCommand
 import qualified Annex
@@ -54,9 +54,6 @@ import qualified Utility.Process
 import qualified Utility.Misc
 import qualified Annex.Content.Direct
 
-import Data.Time.Clock.POSIX
-import System.Posix.Types
-
 -- instances for quickcheck
 instance Arbitrary Types.Key.Key where
 	arbitrary = Types.Key.Key
@@ -76,32 +73,17 @@ instance Arbitrary Logs.Transfer.TransferInfo where
 		<*> arbitrary `suchThat` (/= Just "")
 		<*> arbitrary
 
-instance Arbitrary POSIXTime where
-	arbitrary = abs <$> arbitrarySizedIntegral
-
-instance Arbitrary ProcessID where
-	arbitrary = abs <$> arbitraryBoundedIntegral
-
 instance Arbitrary Annex.Content.Direct.Cache where
 	arbitrary = Annex.Content.Direct.Cache
 		<$> arbitrary
 		<*> arbitrary
 		<*> arbitrary
 
-instance Arbitrary EpochTime where
-	arbitrary = abs <$> arbitrarySizedIntegral
-
-instance Arbitrary FileID where
-	arbitrary = abs <$> arbitrarySizedIntegral
-
-instance Arbitrary FileOffset where
-	arbitrary = abs <$> arbitrarySizedIntegral
-
 instance Arbitrary Logs.Presence.LogLine where
 	arbitrary = Logs.Presence.LogLine
 		<$> arbitrary
 		<*> elements [minBound..maxBound]
-		<*> (arbitrary `suchThat` ('\n' `notElem`))
+		<*> arbitrary `suchThat` ('\n' `notElem`)
 
 main :: IO ()
 main = do
@@ -125,7 +107,6 @@ quickcheck = TestLabel "quickcheck" $ TestList
 	, qctest "prop_idempotent_shellEscape_multiword" Utility.SafeCommand.prop_idempotent_shellEscape_multiword
 	, qctest "prop_idempotent_configEscape" Logs.Remote.prop_idempotent_configEscape
 	, qctest "prop_parentDir_basics" Utility.Path.prop_parentDir_basics
-
 	, qctest "prop_relPathDirToFile_basics" Utility.Path.prop_relPathDirToFile_basics
 	, qctest "prop_relPathDirToFile_regressionTest" Utility.Path.prop_relPathDirToFile_regressionTest
 	, qctest "prop_cost_sane" Config.prop_cost_sane