From 62bd0986c0ff49674d42daee4ff1fdc7074f30e4 Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Wed, 17 Jun 2026 06:28:36 +0000
Subject: [PATCH] Intl: accept legacy IANA primary zones (CET, CST6CDT, EET,
 EST5EDT, MET, MST7MDT, PST8PDT, WET)

These identifiers are first-class `Zone`s in the IANA `etcetera` file
(not backward links), so they are primary IANA time zone identifiers.
ICU reports them as canonical primaries, but
`isValidTimeZoneNameFromICUTimeZone` dropped every non-`/` name except
`UTC`/`GMT`, so `new Intl.DateTimeFormat(undefined, { timeZone: "CET" })`
threw `RangeError` on Linux/Windows. macOS was unaffected (system
libicucore has a different canonical table).

Fixes oven-sh/bun#30618
---
 Source/JavaScriptCore/runtime/IntlObject.cpp | 21 ++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/Source/JavaScriptCore/runtime/IntlObject.cpp b/Source/JavaScriptCore/runtime/IntlObject.cpp
index 7267db91e47e..6e6bb188de0d 100644
--- a/Source/JavaScriptCore/runtime/IntlObject.cpp
+++ b/Source/JavaScriptCore/runtime/IntlObject.cpp
@@ -1951,10 +1951,23 @@ static bool isValidTimeZoneNameFromICUTimeZone(StringView timeZoneName)
         return false;
     if (timeZoneName.startsWith("Etc/"_s))
         return true;
-    // IANA time zone names include '/'. Some of them are not including, but it is in backward links.
-    // And ICU already resolved these backward links.
-    if (!timeZoneName.contains('/'))
-        return timeZoneName == "UTC"_s || timeZoneName == "GMT"_s;
+    // IANA time zone names usually include '/'. The following legacy identifiers
+    // are first-class `Zone`s in the IANA `etcetera` file (not backward links),
+    // so they are primary IANA zones and must be treated as valid:
+    //   CET, CST6CDT, EET, EST5EDT, MET, MST7MDT, PST8PDT, WET
+    // plus UTC/GMT which are always valid.
+    if (!timeZoneName.contains('/')) {
+        return timeZoneName == "UTC"_s
+            || timeZoneName == "GMT"_s
+            || timeZoneName == "CET"_s
+            || timeZoneName == "CST6CDT"_s
+            || timeZoneName == "EET"_s
+            || timeZoneName == "EST5EDT"_s
+            || timeZoneName == "MET"_s
+            || timeZoneName == "MST7MDT"_s
+            || timeZoneName == "PST8PDT"_s
+            || timeZoneName == "WET"_s;
+    }
     return true;
 }