diff --git a/src/SubFix/Internal.hs b/src/SubFix/Internal.hs
index 444bf2b..782f8e0 100644
--- a/src/SubFix/Internal.hs
+++ b/src/SubFix/Internal.hs
@@ -52,6 +52,9 @@ timestamp
   -> Integer
   -- ^ Milliseconds
   -> Integer
-timestamp = undefined
+timestamp h m s ms = let
+  m' = h * 60 + m
+  s' = m' * 60 + s
+  in s' * 1000 + ms
 
 --jl
diff --git a/subfix.cabal b/subfix.cabal
index 0e5e5f8..b6c9c19 100644
--- a/subfix.cabal
+++ b/subfix.cabal
@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: f83e79f4461abbbff9a3ec9abaae7602d26c2384e12b6f2236bc00dcca5ea136
+-- hash: abd53b387d0bc7d910539fc83efe7b2fa75fe55deb728293cb25b78a7475d4cb
 
 name:           subfix
 version:        0.0.0
@@ -57,6 +57,7 @@ test-suite subfix-test
   main-is: Spec.hs
   other-modules:
       SubFix.ConvertSpec
+      SubFix.Internal.TimestampSpec
       SubFix.InternalSpec
       SubFixSpec
       Paths_subfix
diff --git a/test/SubFix/Internal/TimestampSpec.hs b/test/SubFix/Internal/TimestampSpec.hs
new file mode 100644
index 0000000..1234e00
--- /dev/null
+++ b/test/SubFix/Internal/TimestampSpec.hs
@@ -0,0 +1,37 @@
+{-
+
+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.TimestampSpec (spec) where
+
+import Test.Hspec (Spec, describe, it, shouldBe)
+
+import SubFix.Internal (timestamp)
+
+spec :: Spec
+spec = describe "timestamp" $ let
+  expected
+    = 1 * 60 * 60 * 1000
+    + 2 * 60 * 1000
+    + 3 * 1000
+    + 4
+  in it ("should be " ++ show expected) $
+    timestamp 1 2 3 4 `shouldBe` expected
+
+--jl
diff --git a/test/SubFix/InternalSpec.hs b/test/SubFix/InternalSpec.hs
index 1258c46..fb54c24 100644
--- a/test/SubFix/InternalSpec.hs
+++ b/test/SubFix/InternalSpec.hs
@@ -22,7 +22,9 @@ module SubFix.InternalSpec (spec) where
 
 import Test.Hspec (Spec, describe)
 
+import qualified SubFix.Internal.TimestampSpec as Timestamp
+
 spec :: Spec
-spec = describe "Internal" $ return ()
+spec = describe "Internal" Timestamp.spec
 
 --jl