diff --git a/src/SubFix/Internal.hs b/src/SubFix/Internal.hs
index 6c7bfd3..d79d673 100644
--- a/src/SubFix/Internal.hs
+++ b/src/SubFix/Internal.hs
@@ -52,7 +52,15 @@ decodeTime = runStateT $ do
 
 -- | Encodes a timestamp from a number of milliseconds
 encodeTime :: Integer -> String
-encodeTime = undefined
+encodeTime ms = let
+  (s, ms') = ms `divMod` 1000
+  (m, s')  = s `divMod` 60
+  (h, m')  = m `divMod` 60
+  in
+    mkNum 2 h ++ ":" ++
+    mkNum 2 m' ++ ":" ++
+    mkNum 2 s' ++ "," ++
+    mkNum 3 ms'
 
 -- | Converts hours, minutes, seconds and milliseconds into the total
 -- number of milliseconds
@@ -99,4 +107,8 @@ nextChar = get >>= \case
     put str
     return ch
 
+mkNum :: Int -> Integer -> String
+mkNum digits val =
+  reverse $ take digits $ reverse (show val) ++ repeat '0'
+
 --jl
diff --git a/subfix.cabal b/subfix.cabal
index f97865e..7847a88 100644
--- a/subfix.cabal
+++ b/subfix.cabal
@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: ce33ad286b77569617c489030c5bec8597e68ad69f7a34c12d95ad54614a59f2
+-- hash: 0d6580c85ab3588e866980c3fede7319b7d6e699369dc88af2ec75c093c399cf
 
 name:           subfix
 version:        0.0.0
@@ -60,6 +60,7 @@ test-suite subfix-test
       SubFix.ConvertSpec
       SubFix.DecodeSpec
       SubFix.Internal.DecodeTimeSpec
+      SubFix.Internal.EncodeTimeSpec
       SubFix.Internal.TimestampSpec
       SubFix.InternalSpec
       SubFixSpec
diff --git a/test/SubFix/Internal/EncodeTimeSpec.hs b/test/SubFix/Internal/EncodeTimeSpec.hs
new file mode 100644
index 0000000..17ffa82
--- /dev/null
+++ b/test/SubFix/Internal/EncodeTimeSpec.hs
@@ -0,0 +1,41 @@
+{-
+
+subfix
+Copyright (C) Jonathan Lamothe <jonathan@jlamothe.net>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or (at
+your option) any later version.
+
+This program is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+-}
+
+module SubFix.Internal.EncodeTimeSpec (spec) where
+
+import Test.Hspec (Spec, context, describe, it, shouldBe)
+
+import SubFix.Internal (encodeTime, timestamp)
+
+spec :: Spec
+spec = describe "encodeTime" $ mapM_
+  ( \(label, input, expected) ->
+    context label $
+      it ("should be: " ++ expected) $
+        encodeTime input `shouldBe` expected
+  )
+
+  --  label,           input,                  expected
+  [ ( "single digit",  timestamp 1 2 3 4,      "01:02:03,004" )
+  , ( "double digits", timestamp 10 11 12 13,  "10:11:12,013" )
+  , ( "no padding",    timestamp 10 11 12 123, "10:11:12,123" )
+  ]
+
+--jl
diff --git a/test/SubFix/InternalSpec.hs b/test/SubFix/InternalSpec.hs
index dd821cd..d306767 100644
--- a/test/SubFix/InternalSpec.hs
+++ b/test/SubFix/InternalSpec.hs
@@ -23,11 +23,13 @@ module SubFix.InternalSpec (spec) where
 import Test.Hspec (Spec, describe)
 
 import qualified SubFix.Internal.DecodeTimeSpec as DecodeTime
+import qualified SubFix.Internal.EncodeTimeSpec as EncodeTime
 import qualified SubFix.Internal.TimestampSpec as Timestamp
 
 spec :: Spec
 spec = describe "Internal" $ do
   DecodeTime.spec
+  EncodeTime.spec
   Timestamp.spec
 
 --jl