diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright Ashley Yakeley (c) 2016
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Ashley Yakeley nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/leapseconds.cabal b/leapseconds.cabal
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--- /dev/null
+++ b/leapseconds.cabal
@@ -0,0 +1,42 @@
+cabal-version: >=1.10
+name: leapseconds
+version: 1.0
+category: Time
+description: Interpretation of leap second files.
+author: Ashley Yakeley
+license: BSD3
+license-file: LICENSE
+maintainer: ashley@semantic.org
+copyright: 2016 Ashley Yakeley
+build-type: Simple
+
+source-repository head
+    type: git
+    location: https://github.com/AshleyYakeley/leapseconds
+
+library
+    hs-source-dirs: src
+    exposed-modules:
+        Data.Time.Clock.LeapSeconds
+    build-depends:
+        base >= 4.7 && < 5,
+        time >= 1.7
+    default-language: Haskell2010
+    default-extensions:
+    ghc-options: -Wall
+
+test-suite test
+    type: exitcode-stdio-1.0
+    hs-source-dirs: test
+    build-depends:
+        base,
+        time,
+        tasty,
+        tasty-hunit,
+        leapseconds
+    default-language: Haskell2010
+    default-extensions:
+        StandaloneDeriving
+    ghc-options: -Wall
+    main-is: Test.hs
+    other-modules:
diff --git a/src/Data/Time/Clock/LeapSeconds.hs b/src/Data/Time/Clock/LeapSeconds.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Time/Clock/LeapSeconds.hs
@@ -0,0 +1,91 @@
+-- | Interpretation of leap second files.
+module Data.Time.Clock.LeapSeconds
+(
+    LeapSecondMap,
+    LeapSecondList(..),
+    parseNISTLeapSecondList,
+    leapSecondListToMap,
+) where
+{
+    import Data.Maybe;
+    import Text.Read;
+    import Data.Time;
+    import Data.Time.Clock.TAI;
+
+
+    -- | A list of leap-second transitions, etc.
+    ;
+    data LeapSecondList = MkLeapSecondList
+    {
+        lslVersion :: Day, -- when the list was created
+        lslExpiration :: Day, -- when the list expires
+        lslTransitions :: [(Day,Int)] -- transitions: TAI - UTC starting at this day
+    } deriving Eq;
+
+    -- | 1900-01-01
+    ;
+    ntpEpochDay :: Day;
+    ntpEpochDay = ModifiedJulianDay 15020;
+
+    ntpDateStringToDay :: String -> Maybe Day;
+    ntpDateStringToDay s = do
+    {
+        n <- readMaybe s;
+        return $ addDays (div n 86400) ntpEpochDay;
+    };
+
+    single :: [a] -> Maybe a;
+    single [a] = return a;
+    single _ = Nothing;
+
+    separate :: Char -> String -> [String];
+    separate sep s = let
+    {
+        (val,rest) = break ((==) sep) s;
+        vals = case rest of
+        {
+            [] -> [];
+            _:s' -> separate sep s';
+        };
+    } in val : vals;
+
+    -- | Parse the text of a NIST leap-second file. This file can be found at <ftp://time.nist.gov/pub/leap-seconds.list>,
+    -- and on UNIX systems, at @\/usr\/share\/zoneinfo\/leap-seconds.list@.
+    ;
+    parseNISTLeapSecondList :: String -> Maybe LeapSecondList;
+    parseNISTLeapSecondList text = do
+    {
+        let
+        {
+            readLine ('#':'$':s) | Just version <- ntpDateStringToDay s = return (Just version,Nothing,Nothing);
+            readLine ('#':'@':s) | Just expiration <- ntpDateStringToDay s = return (Nothing,Just expiration,Nothing);
+            readLine ('#':_) = return (Nothing,Nothing,Nothing);
+            readLine "" = return (Nothing,Nothing,Nothing);
+            readLine s = case separate '\t' s of
+            {
+                (tstr:ostr:_) | Just t <- ntpDateStringToDay tstr, Just offset <- readMaybe ostr -> return $ (Nothing,Nothing,Just (t,offset));
+                _ -> Nothing;
+            };
+        };
+        mstrs <- traverse readLine $ lines text;
+        version <- single $ catMaybes $ fmap (\(x,_,_) -> x) mstrs;
+        expiration <- single $ catMaybes $ fmap (\(_,x,_) -> x) mstrs;
+        let
+        {
+            transitions = catMaybes $ fmap (\(_,_,x) -> x) mstrs;
+        };
+        return $ MkLeapSecondList version expiration transitions;
+    };
+
+    -- | Obtain a map that can be used to convert between TAI and UTC (see "Data.Time.Clock.TAI")
+    ;
+    leapSecondListToMap :: LeapSecondList -> LeapSecondMap;
+    leapSecondListToMap lsl day | day >= lslExpiration lsl = Nothing;
+    leapSecondListToMap lsl day = let
+    {
+        findInList :: [(Day,Int)] -> Maybe Int -> Maybe Int;
+        findInList [] mi = mi;
+        findInList ((d,_):_) mi | day < d = mi;
+        findInList ((_,i):rest) _ = findInList rest (Just i);
+    } in findInList (lslTransitions lsl) Nothing;
+}
diff --git a/test/Test.hs b/test/Test.hs
new file mode 100644
--- /dev/null
+++ b/test/Test.hs
@@ -0,0 +1,99 @@
+{-# OPTIONS -fno-warn-orphans #-}
+module Main(main) where
+{
+    import Test.Tasty;
+    import Test.Tasty.HUnit;
+    import Data.Time;
+    import Data.Time.Clock.TAI;
+    import Data.Time.Clock.LeapSeconds;
+
+
+    testParseNISTLeapSecondList :: TestTree;
+    testParseNISTLeapSecondList = testCase "parseNISTLeapSecondList" $ let
+    {
+        jan y = fromGregorian y 1 1;
+        jul y = fromGregorian y 7 1;
+
+        refList :: LeapSecondList;
+        refList = MkLeapSecondList
+        {
+            lslVersion = fromGregorian 2016 7 8,
+            lslExpiration = fromGregorian 2017 6 28,
+            lslTransitions =
+            zip
+            [
+                jan 1972,
+                jul 1972,
+                jan 1973,
+                jan 1974,
+                jan 1975,
+                jan 1976,
+                jan 1977,
+                jan 1978,
+                jan 1979,
+                jan 1980,
+                jul 1981,
+                jul 1982,
+                jul 1983,
+                jul 1985,
+                jan 1988,
+                jan 1990,
+                jan 1991,
+                jul 1992,
+                jul 1993,
+                jul 1994,
+                jan 1996,
+                jul 1997,
+                jan 1999,
+                jan 2006,
+                jan 2009,
+                jul 2012,
+                jul 2015,
+                jan 2017
+            ]
+            [10..]
+        };
+    } in do
+    {
+        fileString <- readFile "test/leap-seconds.list";
+        assertEqual "LeapSecondList" (Just refList) $ parseNISTLeapSecondList fileString;
+    };
+
+    deriving instance Show LeapSecondList;
+
+    testLeapSecondListToMap :: TestTree;
+    testLeapSecondListToMap = testCase "leapSecondListToMap" $ let
+    {
+        sampleLeapSecondList :: LeapSecondList;
+        sampleLeapSecondList = MkLeapSecondList
+        {
+            lslVersion = fromGregorian 1974 1 1,
+            lslExpiration = fromGregorian 1975 1 1,
+            lslTransitions =
+            [
+                (fromGregorian 1972 1 1,10),
+                (fromGregorian 1972 7 1,11)
+            ]
+        };
+
+        sampleLeapSecondMap :: LeapSecondMap;
+        sampleLeapSecondMap = leapSecondListToMap sampleLeapSecondList;
+
+        assertVal expected day = assertEqual (show day) expected $ sampleLeapSecondMap day;
+    } in do
+    {
+        assertVal Nothing (fromGregorian 1971 12 31);
+        assertVal (Just 10) (fromGregorian 1972 1 1);
+        assertVal (Just 10) (fromGregorian 1972 6 31);
+        assertVal (Just 11) (fromGregorian 1972 7 1);
+        assertVal (Just 11) (fromGregorian 1974 12 31);
+        assertVal Nothing (fromGregorian 1975 1 1);
+    };
+
+    main :: IO ();
+    main = defaultMain $ testGroup "leapseconds" $
+    [
+        testParseNISTLeapSecondList,
+        testLeapSecondListToMap
+    ];
+}