fix(js/ts): reject JS-permissive date strings that .NET TryParse rejects

V8's Date constructor accepts strings like "ABC 6" by treating the
letter sequence as a timezone abbreviation. .NET DateTime.TryParse
rejects such strings as invalid.

Add a pre-validation check in parseRaw() that requires the input to
start with a digit or a recognised month-name prefix (Jan–Dec).
This matches .NET behaviour while preserving all existing valid formats.

Closes #3858

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

src/fable-library-ts/Date.ts

@@ -599,6 +599,15 @@ export function parseRaw(input: string): [Date, Offset] {
     fail();
   }
 
+  // Pre-validate: reject strings that JavaScript's Date constructor accepts but .NET does not.
+  // JS Date (V8) is overly permissive — e.g. "ABC 6" is accepted because V8 treats "ABC" as a
+  // timezone abbreviation. .NET DateTime.TryParse requires a recognisable date format.
+  // Valid date strings must start with a digit, or with a recognised month name (Jan–Dec).
+  // See #3858.
+  if (!/^\s*(?:\d|Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)/i.test(input)) {
+    fail();
+  }
+
   // ISO dates without TZ are parsed as UTC. Adding time without TZ keeps them local.
   if (input.length === 10 && input[4] === "-" && input[7] === "-") {
     input += "T00:00:00";

tests/Js/Main/DateTimeTests.fs

@@ -876,6 +876,20 @@ let tests =
         let r, _date = DateTime.TryParse(invalidAmericanDate, CultureInfo.InvariantCulture, DateTimeStyles.None)
         r |> equal false
 
+    testCase "DateTime.TryParse rejects JS-permissive strings that .NET rejects" <| fun () ->
+        // V8's Date constructor accepts "ABC 6" by treating "ABC" as a timezone abbreviation.
+        // .NET DateTime.TryParse must reject it. See #3858.
+        let r1, _ = DateTime.TryParse("ABC 6")
+        r1 |> equal false
+
+        let r2, _ = DateTime.TryParse("XYZ 2024")
+        r2 |> equal false
+
+        // Valid dates must still parse
+        let r3, d = DateTime.TryParse("9/10/2014 1:50:34 PM")
+        r3 |> equal true
+        d.Year |> equal 2014
+
     testCase "DateTime.Today works" <| fun () ->
         let d = DateTime.Today
         equal 0 d.Hour