Fix toLocaleString timeZoneName returning full name instead of abbreviation

Jint.Tests/Runtime/DateTests.cs

@@ -114,6 +114,44 @@ public void ToStringUsesDaylightNameWhenInDst()
         Assert.Contains("(Eastern Standard Time)", engine.Evaluate("new Date(2022, 0, 4).toString()").AsString());
     }
 
+    [Fact]
+    public void ToLocaleStringUsesShortTimeZoneAbbreviation()
+    {
+        TimeZoneInfo timeZoneInfo;
+        try
+        {
+            timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
+        }
+        catch (TimeZoneNotFoundException)
+        {
+            timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("America/New_York");
+        }
+
+        var engine = new Engine(options => options.LocalTimeZone(timeZoneInfo));
+
+        const string script = """
+            (function() {
+                var d = new Date(2022, 6, 4); // July 4, 2022 - DST (EDT)
+                return d.toLocaleString('en-US', { timeZoneName: 'short' });
+            })();
+            """;
+
+        // Should return "EDT" abbreviation, not "Eastern Daylight Time"
+        var result = engine.Evaluate(script).AsString();
+        Assert.Contains("EDT", result);
+
+        const string scriptWinter = """
+            (function() {
+                var d = new Date(2022, 0, 4); // January 4, 2022 - standard time (EST)
+                return d.toLocaleString('en-US', { timeZoneName: 'short' });
+            })();
+            """;
+
+        // Should return "EST" abbreviation, not "Eastern Standard Time"
+        var resultWinter = engine.Evaluate(scriptWinter).AsString();
+        Assert.Contains("EST", resultWinter);
+    }
+
     [Theory]
     [InlineData("Thu, 30 Jan 2020 08:00:00 PST", 1580400000000)]
     [InlineData("Thursday January 01 1970 00:00:25 UTC", 25000)]

Jint/Native/Intl/Data/MetaZoneData.cs

@@ -515,6 +515,100 @@ internal readonly record struct MetaZoneNames(
 
         // Apia (Pacific/Apia mapped above under Samoa section - current metazone is Apia)
 
+        // Windows timezone ID aliases (for .NET Framework compatibility where TryConvertWindowsIdToIanaId is unavailable)
+        // These map Windows timezone IDs directly to metazone names.
+        // America
+        ["Eastern Standard Time"] = "America_Eastern",
+        ["US Eastern Standard Time"] = "America_Eastern",
+        ["Central Standard Time"] = "America_Central",
+        ["US Central Standard Time"] = "America_Central",
+        ["Canada Central Standard Time"] = "America_Central",
+        ["Central America Standard Time"] = "America_Central",
+        ["Mexico Standard Time"] = "America_Central",
+        ["Mountain Standard Time"] = "America_Mountain",
+        ["US Mountain Standard Time"] = "America_Mountain",
+        ["Pacific Standard Time"] = "America_Pacific",
+        ["Pacific Standard Time (Mexico)"] = "America_Pacific",
+        ["Alaskan Standard Time"] = "Alaska",
+        ["Hawaiian Standard Time"] = "Hawaii_Aleutian",
+        ["Aleutian Standard Time"] = "Hawaii_Aleutian",
+        ["Atlantic Standard Time"] = "Atlantic",
+        ["Turks And Caicos Standard Time"] = "Atlantic",
+        ["Newfoundland Standard Time"] = "Newfoundland",
+        ["Greenland Standard Time"] = "Greenland_Western",
+        ["E. South America Standard Time"] = "Brasilia",
+        ["Bahia Standard Time"] = "Brasilia",
+        ["Central Brazilian Standard Time"] = "Amazon",
+        ["SA Western Standard Time"] = "Amazon",
+        ["Argentina Standard Time"] = "Argentina",
+        ["SA Eastern Standard Time"] = "Brasilia",
+        ["SA Pacific Standard Time"] = "Colombia",
+        ["Venezuela Standard Time"] = "Venezuela",
+        ["Paraguay Standard Time"] = "Paraguay",
+        ["Uruguay Standard Time"] = "Uruguay",
+        ["Cuba Standard Time"] = "Cuba",
+        // Europe
+        ["GMT Standard Time"] = "GMT",
+        ["Greenwich Standard Time"] = "GMT",
+        ["W. Europe Standard Time"] = "Europe_Central",
+        ["Central Europe Standard Time"] = "Europe_Central",
+        ["Central European Standard Time"] = "Europe_Central",
+        ["Romance Standard Time"] = "Europe_Central",
+        ["W. Central Africa Standard Time"] = "Africa_Western",
+        ["GTB Standard Time"] = "Europe_Eastern",
+        ["FLE Standard Time"] = "Europe_Eastern",
+        ["E. Europe Standard Time"] = "Europe_Eastern",
+        ["Turkey Standard Time"] = "Europe_Eastern",
+        ["Russian Standard Time"] = "Moscow",
+        ["Belarus Standard Time"] = "Moscow",
+        // Asia
+        ["China Standard Time"] = "China",
+        ["Taipei Standard Time"] = "Taipei",
+        ["Tokyo Standard Time"] = "Japan",
+        ["Korea Standard Time"] = "Korea",
+        ["Singapore Standard Time"] = "Singapore",
+        ["India Standard Time"] = "India",
+        ["Pakistan Standard Time"] = "Pakistan",
+        ["Bangladesh Standard Time"] = "Bangladesh",
+        ["SE Asia Standard Time"] = "Indochina",
+        ["Arabian Standard Time"] = "Gulf",
+        ["Arab Standard Time"] = "Arabian",
+        ["Arabic Standard Time"] = "Arabian",
+        ["Iran Standard Time"] = "Iran",
+        ["Israel Standard Time"] = "Israel",
+        ["W. Asia Standard Time"] = "Uzbekistan",
+        ["Central Asia Standard Time"] = "Kazakhstan_Eastern",
+        ["Ekaterinburg Standard Time"] = "Yekaterinburg",
+        ["N. Central Asia Standard Time"] = "Novosibirsk",
+        ["North Asia Standard Time"] = "Krasnoyarsk",
+        ["North Asia East Standard Time"] = "Irkutsk",
+        ["Yakutsk Standard Time"] = "Yakutsk",
+        ["Russia Time Zone 8"] = "Yakutsk",
+        ["Vladivostok Standard Time"] = "Vladivostok",
+        ["Magadan Standard Time"] = "Magadan",
+        ["Kamchatka Standard Time"] = "Kamchatka",
+        ["Myanmar Standard Time"] = "Myanmar",
+        ["Afghanistan Standard Time"] = "Afghanistan",
+        ["Nepal Standard Time"] = "Nepal",
+        ["Sri Lanka Standard Time"] = "Lanka",
+        ["Malaysia Standard Time"] = "Malaysia",
+        ["West Asia Standard Time"] = "Pakistan",
+        // Australia/Pacific
+        ["AUS Eastern Standard Time"] = "Australia_Eastern",
+        ["E. Australia Standard Time"] = "Australia_Eastern",
+        ["Tasmania Standard Time"] = "Australia_Eastern",
+        ["Cen. Australia Standard Time"] = "Australia_Central",
+        ["AUS Central Standard Time"] = "Australia_Central",
+        ["W. Australia Standard Time"] = "Australia_Western",
+        ["New Zealand Standard Time"] = "New_Zealand",
+        ["Fiji Standard Time"] = "Fiji",
+        ["Tonga Standard Time"] = "Tonga",
+        ["Samoa Standard Time"] = "Apia",
+        // Africa
+        ["South Africa Standard Time"] = "Africa_Southern",
+        ["E. Africa Standard Time"] = "Africa_Eastern",
+        ["Egypt Standard Time"] = "Africa_Eastern",
+
     };
 
     /// <summary>

Jint/Native/Intl/JsDateTimeFormat.cs

@@ -1505,7 +1505,50 @@ private string GetTimeZoneDisplayName(DateTime utcDateTime, bool longName, bool
                 return longName ? TimeZone : parts[parts.Length - 1];
             }
         }
-        return longName ? TimeZoneInfo.Local.StandardName : TimeZoneInfo.Local.Id;
+        // No explicit timezone: use the engine's default timezone
+        var defaultTz = _engine.Options.TimeSystem.DefaultTimeZone;
+
+        var defaultIsDst = false;
+        try
+        {
+            defaultIsDst = defaultTz.IsDaylightSavingTime(DateTime.SpecifyKind(utcDateTime, DateTimeKind.Utc));
+        }
+        catch
+        {
+            // If DST check fails, defaultIsDst stays false
+        }
+
+        // Try to get an IANA timezone ID for the CLDR lookup.
+        // The IanaToMetaZone table also includes Windows timezone ID aliases for .NET Framework compatibility.
+        var defaultTzId = defaultTz.Id;
+#if NET6_0_OR_GREATER
+        if (!defaultTz.HasIanaId && TimeZoneInfo.TryConvertWindowsIdToIanaId(defaultTzId, out var defaultIanaId))
+        {
+            defaultTzId = defaultIanaId;
+        }
+#endif
+
+        var defaultCldrName = Data.MetaZoneData.GetDisplayName(defaultTzId, defaultIsDst, longName, generic);
+        if (defaultCldrName != null)
+        {
+            return defaultCldrName;
+        }
+
+        // Fallback to .NET TimeZoneInfo names
+        if (longName)
+        {
+            return defaultIsDst ? defaultTz.DaylightName : defaultTz.StandardName;
+        }
+
+        // For short names: try to parse an abbreviation from IANA-style IDs (e.g. "America/New_York" → "New York")
+        // Windows timezone IDs don't contain '/', so fall back to DST-aware names
+        if (defaultTzId.Contains('/'))
+        {
+            var defaultParts = defaultTzId.Split('/');
+            return defaultParts[defaultParts.Length - 1].Replace('_', ' ');
+        }
+
+        return defaultIsDst ? defaultTz.DaylightName : defaultTz.StandardName;
     }
 
     /// <summary>