From e8f04f054fbc58ddd500e55b001df88b1d22b494 Mon Sep 17 00:00:00 2001
From: "fable-repo-assist[bot]"
 <279220273+fable-repo-assist[bot]@users.noreply.github.com>
Date: Sun, 3 May 2026 01:31:58 +0000
Subject: [PATCH 1/2] fix(js/ts): reject JS-permissive date strings that .NET
 TryParse rejects
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
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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>
---
 src/fable-library-ts/Date.ts   |  9 +++++++++
 tests/Js/Main/DateTimeTests.fs | 14 ++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/src/fable-library-ts/Date.ts b/src/fable-library-ts/Date.ts
index ccf50fce6..cec74789e 100644
--- a/src/fable-library-ts/Date.ts
+++ b/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";
diff --git a/tests/Js/Main/DateTimeTests.fs b/tests/Js/Main/DateTimeTests.fs
index 8fd2a1572..9f96744da 100644
--- a/tests/Js/Main/DateTimeTests.fs
+++ b/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

From 64c98acc40d444b9877346393ecaf858ed849e79 Mon Sep 17 00:00:00 2001
From: Mangel Maxime <me@mangelmaxime.fr>
Date: Fri, 29 May 2026 19:13:27 +0200
Subject: [PATCH 2/2] fix: implementation

---
 src/fable-library-ts/Date.ts   | 19 +++++++++++++------
 tests/Js/Main/DateTimeTests.fs | 33 ++++++++++++++++++++++-----------
 2 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/src/fable-library-ts/Date.ts b/src/fable-library-ts/Date.ts
index cec74789e..968dcbb68 100644
--- a/src/fable-library-ts/Date.ts
+++ b/src/fable-library-ts/Date.ts
@@ -590,6 +590,18 @@ export function maxValue() {
   return DateTime(253402300799999, DateTimeKind.Utc);
 }
 
+// The only date words .NET's invariant parser recognises: month names, weekday names,
+// meridiem designators and zone markers (plus the ISO "T" separator). Anything else is
+// rejected. Used to reject JS-permissive inputs (see `parseRaw`).
+const recognizedDateWords = new Set([
+  "january", "february", "march", "april", "may", "june",
+  "july", "august", "september", "october", "november", "december",
+  "jan", "feb", "mar", "apr", "jun", "jul", "aug", "sep", "sept", "oct", "nov", "dec",
+  "monday", "tuesday", "wednesday", "thursday", "friday", "saturday", "sunday",
+  "mon", "tue", "wed", "thu", "fri", "sat", "sun",
+  "am", "pm", "gmt", "utc", "ut", "t", "z",
+]);
+
 export function parseRaw(input: string): [Date, Offset] {
   function fail() {
     throw new Exception(`The string is not a valid Date: ${input}`);
@@ -599,12 +611,7 @@ 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)) {
+  if ((input.match(/[a-z]+/gi) ?? []).some(word => !recognizedDateWords.has(word.toLowerCase()))) {
     fail();
   }
 
diff --git a/tests/Js/Main/DateTimeTests.fs b/tests/Js/Main/DateTimeTests.fs
index 9f96744da..0cdc25741 100644
--- a/tests/Js/Main/DateTimeTests.fs
+++ b/tests/Js/Main/DateTimeTests.fs
@@ -877,18 +877,29 @@ let tests =
         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")
+        // JS's Date constructor treats unrecognised words as timezone abbreviations, so it
+        // accepts strings .NET rejects: "ABC 6" (ABC), "XYZ 2024" (XYZ), and words that merely
+        // start with a month name like "Maybe 6" (May) or "Junk 2024" (Jun).
+        [ "ABC 6"; "XYZ 2024"; "Maybe 6"; "Junk 2024" ]
+        |> List.map (fun s -> fst (DateTime.TryParse s))
+        |> equal [ false; false; false; false ]
+
+        // Recognised date words (weekday / month names, GMT) must still parse, matching .NET.
+        let r1, d1 = DateTime.TryParse("Sun, 06 Nov 1994 08:49:37 GMT")
+        r1 |> equal true
+        d1.Year |> equal 1994
+
+        let r2, d2 = DateTime.TryParse("Mon, 15 Jan 2024")
+        r2 |> equal true
+        d2.Year |> equal 2024
+
+        let r3, d3 = DateTime.TryParse("January 15, 2024")
         r3 |> equal true
-        d.Year |> equal 2014
+        d3.Year |> equal 2024
+
+        let r4, d4 = DateTime.TryParse("9/10/2014 1:50:34 PM")
+        r4 |> equal true
+        d4.Year |> equal 2014
 
     testCase "DateTime.Today works" <| fun () ->
         let d = DateTime.Today