diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index f227b18..e4aee13 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -69,11 +69,11 @@ jobs:
           path: |
             backend/test/functional/goldens
 
+      # yamllint disable rule:line-length
       - name: Generate charts json with backend
         run: |
-          cabal run pacer -- chart \
-            --runs backend/data/input/example/runs.toml \
-            --chart-requests backend/data/input/example/chart-requests.toml
+          cabal run pacer -- chart --runs backend/data/input/example/runs.toml --chart-requests backend/data/input/example/chart-requests.toml
+      # yamllint enable rule:line-length
 
       - name: Install web dependencies
         run: |
diff --git a/backend/pacer.cabal b/backend/pacer.cabal
index 1d6f076..f56505b 100644
--- a/backend/pacer.cabal
+++ b/backend/pacer.cabal
@@ -35,6 +35,15 @@ common common-lang
     TypeAbstractions
     TypeFamilyDependencies
 
+  if os(windows)
+    cpp-options: -DWINDOWS
+
+  elif os(osx)
+    cpp-options: -DOSX -DPOSIX
+
+  else
+    cpp-options: -DLINUX -DPOSIX
+
   default-language:   GHC2021
 
 library
diff --git a/backend/src/Pacer/Prelude.hs b/backend/src/Pacer/Prelude.hs
index 90f792e..892d8d3 100644
--- a/backend/src/Pacer/Prelude.hs
+++ b/backend/src/Pacer/Prelude.hs
@@ -63,6 +63,13 @@ module Pacer.Prelude
     -- * Dev / Debug
     todo,
 
+    -- * OS
+    Os (..),
+    currentOs,
+    currentOsStr,
+    isPosix,
+    posixWindowsStr,
+
     -- * Misc
     pattern SetToSeqNE,
     errorLeft,
@@ -112,6 +119,7 @@ import Data.Bool as X (Bool (False, True), not, otherwise, (&&), (||))
 import Data.ByteString as X (ByteString)
 import Data.ByteString.Lazy qualified as BSL
 import Data.Char as X (Char)
+import Data.Char qualified as Ch
 import Data.Either as X (Either (Left, Right))
 import Data.Eq as X (Eq ((/=), (==)))
 import Data.Foldable as X
@@ -393,3 +401,32 @@ knownExceptions :: List ExceptionProxy
 knownExceptions =
   [ MkExceptionProxy @TextException Proxy
   ]
+
+data Os
+  = Linux
+  | Osx
+  | Windows
+  deriving stock (Eq, Show)
+
+currentOs :: Os
+#if WINDOWS
+currentOs = Windows
+#elif OSX
+currentOs = Osx
+#else
+currentOs = Linux
+#endif
+
+currentOsStr :: String
+currentOsStr = Ch.toLower <$> show currentOs
+
+isPosix :: Bool
+isPosix = case currentOs of
+  Windows -> False
+  _ -> True
+
+posixWindowsStr :: String
+posixWindowsStr =
+  if isPosix
+    then "posix"
+    else "windows"
diff --git a/backend/test/functional/Functional/Chart.hs b/backend/test/functional/Functional/Chart.hs
index 2618470..11de40e 100644
--- a/backend/test/functional/Functional/Chart.hs
+++ b/backend/test/functional/Functional/Chart.hs
@@ -50,6 +50,6 @@ testFilterEmptyError = testChart desc [osp|testFilterEmptyError|]
     desc = "Filter empty error"
 
 testDuplicateDateError :: IO OsPath -> TestTree
-testDuplicateDateError = testChart desc [osp|testDuplicateDateError|]
+testDuplicateDateError = testChartPosix True desc [osp|testDuplicateDateError|]
   where
     desc = "Duplicate date error"
diff --git a/backend/test/functional/Functional/Prelude.hs b/backend/test/functional/Functional/Prelude.hs
index 21ccf43..935c6bd 100644
--- a/backend/test/functional/Functional/Prelude.hs
+++ b/backend/test/functional/Functional/Prelude.hs
@@ -16,11 +16,12 @@ module Functional.Prelude
     GoldenParams (..),
     testGoldenParams,
     testChart,
+    testChartPosix,
   )
 where
 
 import Data.Word (Word8)
-import FileSystem.OsPath (unsafeDecode)
+import FileSystem.OsPath (unsafeDecode, unsafeEncode)
 import FileSystem.OsPath qualified as FS.OsPath
 import Hedgehog as X
   ( Gen,
@@ -129,8 +130,31 @@ data GoldenParams = MkGoldenParams
 --
 -- testFoo is the 'test name'.
 testChart :: TestName -> OsPath -> IO OsPath -> TestTree
-testChart testDesc testName getTestDir = testGoldenParams getTestDir params
+testChart = testChartPosix False
+
+-- | Like 'testChart', except it includes to determine if we take the current
+-- OS into account.
+testChartPosix ::
+  -- | If true, we will append posix/windows to the end of the golden test
+  -- path e.g. testName_posix.golden.
+  Bool ->
+  -- | Test description.
+  TestName ->
+  -- | Test name.
+  OsPath ->
+  -- | Retrieves the current directory.
+  IO OsPath ->
+  TestTree
+testChartPosix osSwitch testDesc testName getTestDir = testGoldenParams getTestDir params
   where
+    -- This is the path to the golden files. If osSwitch is false, then it
+    -- is the same as testName e.g. testName.golden. If the switch is active,
+    -- then we need to append the os e.g. testName_posix.golden.
+    goldenName =
+      if osSwitch
+        then testName <> unsafeEncode ("_" ++ posixWindowsStr)
+        else testName
+
     params =
       MkGoldenParams
         { mkArgs = \testDir ->
@@ -143,7 +167,7 @@ testChart testDesc testName getTestDir = testGoldenParams getTestDir params
               unsafeDecode (mkJsonPath testDir)
             ],
           testDesc,
-          testName,
+          testName = goldenName,
           -- NOTE: It would be nice to test the txt output here i.e. the
           -- second arg. Alas, it includes the path of the output json file,
           -- which is non-deterministic, as it includes the tmp dir.
@@ -153,6 +177,8 @@ testChart testDesc testName getTestDir = testGoldenParams getTestDir params
           resultToBytes = \path _ -> readBinaryFileIO . mkJsonPath $ path
         }
 
+    -- These are always based on the testName, since all Os's share the same
+    -- CLI inputs.
     basePath = [ospPathSep|test/functional/data|]
     chartRequestsPath = unsafeDecode $ basePath </> testName <> [osp|_chart-requests.toml|]
     runsPath = unsafeDecode $ basePath </> testName <> [osp|_runs.toml|]
diff --git a/backend/test/functional/goldens/testDuplicateDateError.golden b/backend/test/functional/goldens/testDuplicateDateError_posix.golden
similarity index 100%
rename from backend/test/functional/goldens/testDuplicateDateError.golden
rename to backend/test/functional/goldens/testDuplicateDateError_posix.golden
diff --git a/backend/test/functional/goldens/testDuplicateDateError_windows.golden b/backend/test/functional/goldens/testDuplicateDateError_windows.golden
new file mode 100644
index 0000000..ef75941
--- /dev/null
+++ b/backend/test/functional/goldens/testDuplicateDateError_windows.golden
@@ -0,0 +1,3 @@
+Error decoding toml file 'test\functional\data\testDuplicateDateError_runs.toml': Decode error at '': Found overlapping timestamps
+ - <no title>: 2024-10-20T14:30:00
+ - A 5k: 2024-10-20
diff --git a/web/src/build_charts.ts b/web/src/build_charts.ts
index 3295a11..6a23b66 100644
--- a/web/src/build_charts.ts
+++ b/web/src/build_charts.ts
@@ -15,21 +15,22 @@ function mkDataSet(x: { values: number[]; label: string }) {
  * We widen charts to charts_typed since if charts.json does not have any
  * charts with a y1 axis, ts will infer the y1 prop does not exist,
  * hence the y1 access will fail.
-*/
+ */
 const charts_typed = charts as {
   data: {
     x: string[];
     y: {
-      label: string; values: number[];
+      label: string;
+      values: number[];
     };
     y1: {
-      label: string; values: number[];
+      label: string;
+      values: number[];
     };
   };
   title: string;
 }[];
 
-
 for (var i = 0; i <= charts_typed.length; i++) {
   const elemId = `chart${i}`;
   appendCanvasId(elemId);