diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 0.3.4.0
+
+- Add `NominalDiffTime` instances
+
 # 0.3.3.0
 
 - Enable `PolyKinds` to generalize `Proxy`, `Tagged`, and `Const` instances.
diff --git a/aeson-compat.cabal b/aeson-compat.cabal
--- a/aeson-compat.cabal
+++ b/aeson-compat.cabal
@@ -1,9 +1,9 @@
--- This file has been generated from package.yaml by hpack version 0.13.0.
+-- This file has been generated from package.yaml by hpack version 0.14.0.
 --
 -- see: https://github.com/sol/hpack
 
 name:           aeson-compat
-version:        0.3.3.0
+version:        0.3.4.0
 synopsis:       Compatibility layer for aeson
 description:    Compatibility layer for @aeson@
 category:       Web
diff --git a/src/Data/Aeson/Compat.hs b/src/Data/Aeson/Compat.hs
--- a/src/Data/Aeson/Compat.hs
+++ b/src/Data/Aeson/Compat.hs
@@ -61,7 +61,7 @@
 import           Data.Typeable (Typeable)
 
 #if !MIN_VERSION_aeson(0,10,0)
-import           Data.Time (Day, LocalTime, formatTime)
+import           Data.Time (Day, LocalTime, formatTime, NominalDiffTime)
 import           Data.Time.Locale.Compat (defaultTimeLocale)
 import qualified Data.Aeson.Compat.Time as CompatTime
 #endif
@@ -221,6 +221,23 @@
 
 instance ToJSON LocalTime where
   toJSON = toJSON . T.pack . formatTime defaultTimeLocale "%FT%T%Q"
+
+instance ToJSON NominalDiffTime where
+  toJSON = Number . realToFrac
+  {-# INLINE toJSON #-}
+	
+#if MIN_VERSION_aeson(0,10,0)
+  toEncoding = Encoding . E.number . realToFrac
+  {-# INLINE toEncoding #-}
+#endif
+	
+-- | /WARNING:/ Only parse lengths of time from trusted input
+-- since an attacker could easily fill up the memory of the target
+-- system by specifying a scientific number with a big exponent like
+-- @1e1000000000@.
+instance FromJSON NominalDiffTime where
+  parseJSON = withScientific "NominalDiffTime" $ pure . realToFrac
+  {-# INLINE parseJSON #-}
 #endif
 
 -----------------------------------------------------------------------
diff --git a/test/Tests.hs b/test/Tests.hs
--- a/test/Tests.hs
+++ b/test/Tests.hs
@@ -9,7 +9,7 @@
 import Data.List.NonEmpty        (NonEmpty)
 import Data.Proxy                (Proxy)
 import Data.Tagged               (Tagged)
-import Data.Time                 (Day, LocalTime)
+import Data.Time                 (Day, LocalTime, NominalDiffTime)
 import Data.Version              (Version)
 import Numeric.Natural           (Natural)
 
@@ -37,6 +37,7 @@
     , testProperty "Proxy"     $ roundtrip (undefined :: Proxy Int)
     , testProperty "Tagged"    $ roundtrip (undefined :: Tagged Int Int)
     , testProperty "NonEmpty"  $ roundtrip (undefined :: NonEmpty Int)
+    , testProperty "NominalDiffTime" $ roundtrip (undefined :: NominalDiffTime)
     ]
   ]