Fix packaged Fable date parsing

Author: adz

scripts/check-fable-package-compat.sh

@@ -11,3 +11,4 @@ cd "$repo_root"
 rm -rf "$out_dir" "$package_cache"
 dotnet pack src/CodecMapper/CodecMapper.fsproj --nologo -v minimal
 dotnet tool run fable -- tests/CodecMapper.FablePackageTests -o "$out_dir" --noRestore --silent
+node "$out_dir/Program.js"

scripts/check-fable5-package-compat.sh

@@ -16,3 +16,4 @@ dotnet tool install fable --version 5.0.0-rc.2 --tool-path "$tool_dir"
 
 DOTNET_ROOT="$dotnet_root" PATH="$dotnet_root:$PATH" \
     "$tool_dir/fable" tests/CodecMapper.FablePackageTests -o "$out_dir" --noRestore --silent
+node "$out_dir/Program.js"

src/CodecMapper/Core.fs

@@ -148,6 +148,40 @@ module Core =
     let tryParseFloatInvariant = tryParseFloatPlatformInvariant
     let tryParseDecimalInvariant = tryParseDecimalPlatformInvariant
 
+    ///
+    /// Fable's packaged `fable-library-js` surface does not currently expose the
+    /// `ParseExact` helpers for date/time types, so the shared schemas need one
+    /// portable entry point that preserves exact parsing on .NET and compatible
+    /// parsing on Fable.
+    let parseDateTimeRoundtripInvariant (text: string) =
+#if FABLE_COMPILER
+        System.DateTime.Parse(text)
+#else
+        System.DateTime.ParseExact(text, "O", CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind)
+#endif
+
+    ///
+    /// The package consumer path exercises the published Fable runtime rather
+    /// than the local source build, so `DateTimeOffset` parsing needs the same
+    /// fallback to avoid importing helpers that the package does not ship.
+    let parseDateTimeOffsetRoundtripInvariant (text: string) =
+#if FABLE_COMPILER
+        System.DateTimeOffset.Parse(text)
+#else
+        System.DateTimeOffset.ParseExact(text, "O", CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind)
+#endif
+
+    ///
+    /// `TimeSpan.ParseExact` hits the same missing-helper problem under Fable,
+    /// while plain invariant parsing still accepts the canonical `"c"` payloads
+    /// that the encoder emits.
+    let parseTimeSpanConstantInvariant (text: string) =
+#if FABLE_COMPILER
+        System.TimeSpan.Parse(text)
+#else
+        System.TimeSpan.ParseExact(text, "c", CultureInfo.InvariantCulture)
+#endif
+
     let private failIntegerToken typeName allowMinus token =
         match classifyIntegerToken allowMinus token with
         | OutOfRangeToken -> failwithf "%s value out of range: %s" typeName token

src/CodecMapper/Schema.fs

@@ -235,29 +235,17 @@ module Schema =
     /// A schema for `DateTime` using the round-trippable `"O"` string format.
     let dateTime: Schema<System.DateTime> =
         string
-        |> map
-            (fun value ->
-                System.DateTime.ParseExact(value, "O", CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind))
-            (fun value -> value.ToString("O", CultureInfo.InvariantCulture))
+        |> map parseDateTimeRoundtripInvariant (fun value -> value.ToString("O", CultureInfo.InvariantCulture))
 
     /// A schema for `DateTimeOffset` using the round-trippable `"O"` string format.
     let dateTimeOffset: Schema<System.DateTimeOffset> =
         string
-        |> map
-            (fun value ->
-                System.DateTimeOffset.ParseExact(
-                    value,
-                    "O",
-                    CultureInfo.InvariantCulture,
-                    DateTimeStyles.RoundtripKind
-                ))
-            (fun value -> value.ToString("O", CultureInfo.InvariantCulture))
+        |> map parseDateTimeOffsetRoundtripInvariant (fun value -> value.ToString("O", CultureInfo.InvariantCulture))
 
     /// A schema for `TimeSpan` using the invariant `"c"` format.
     let timeSpan: Schema<System.TimeSpan> =
         string
-        |> map (fun value -> System.TimeSpan.ParseExact(value, "c", CultureInfo.InvariantCulture)) (fun value ->
-            value.ToString("c", CultureInfo.InvariantCulture))
+        |> map parseTimeSpanConstantInvariant (fun value -> value.ToString("c", CultureInfo.InvariantCulture))
 
     ///
     /// Keep the .NET path using round-trip float formatting, but let Fable use