[release/6.0][iOS] Grab the default timezone using native API

src/libraries/Native/Unix/System.Native/CMakeLists.txt

@@ -46,6 +46,11 @@ else()
     list (APPEND NATIVE_SOURCES pal_environment.c)
 endif()
 
+if (CLR_CMAKE_TARGET_MACCATALYST OR CLR_CMAKE_TARGET_IOS OR CLR_CMAKE_TARGET_TVOS)
+    set(NATIVE_SOURCES ${NATIVE_SOURCES}
+        pal_datetime.m)
+endif()
+
 if (CLR_CMAKE_TARGET_MACCATALYST OR CLR_CMAKE_TARGET_IOS OR CLR_CMAKE_TARGET_TVOS)
     set(NATIVE_SOURCES ${NATIVE_SOURCES}
         pal_log.m

src/libraries/Native/Unix/System.Native/pal_datetime.h

@@ -8,6 +8,6 @@
 
 PALEXPORT int64_t SystemNative_GetSystemTimeAsTicks(void);
 
-#if defined(TARGET_ANDROID)
+#if defined(TARGET_ANDROID) || defined(__APPLE__)
 PALEXPORT char* SystemNative_GetDefaultTimeZone(void);
 #endif

src/libraries/Native/Unix/System.Native/pal_datetime.m

@@ -0,0 +1,12 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#include "pal_datetime.h"
+#import <Foundation/Foundation.h>
+
+char* SystemNative_GetDefaultTimeZone()
+{
+    NSTimeZone *tz = [NSTimeZone localTimeZone];
+    NSString *name = [tz name];
+    return (name != nil) ? strdup([name UTF8String]) : NULL;
+}

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -1954,8 +1954,8 @@
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.GetCwd.cs">
       <Link>Common\Interop\Unix\System.Native\Interop.GetCwd.cs</Link>
     </Compile>
-    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.GetDefaultTimeZone.Android.cs" Condition="'$(TargetsAndroid)' == 'true'">
-      <Link>Common\Interop\Unix\System.Native\Interop.GetDefaultTimeZone.Android.cs</Link>
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.GetDefaultTimeZone.AnyMobile.cs" Condition="'$(TargetsAndroid)' == 'true' or '$(IsiOSLike)' == 'true'">
+      <Link>Common\Interop\Unix\System.Native\Interop.GetDefaultTimeZone.AnyMobile.cs</Link>
     </Compile>
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.GetEGid.cs">
       <Link>Common\Interop\Unix\System.Native\Interop.GetEGid.cs</Link>

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.Unix.NonAndroid.cs

@@ -5,8 +5,9 @@
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
-using System.Text;
+using System.Runtime.InteropServices;
 using System.Security;
+using System.Text;
 using Microsoft.Win32.SafeHandles;
 
 namespace System
@@ -382,6 +383,13 @@ private static bool TryLoadTzFile(string tzFilePath, [NotNullWhen(true)] ref byt
 
         /// <summary>
         /// Gets the tzfile raw data for the current 'local' time zone using the following rules.
+        ///
+        /// On iOS / tvOS
+        /// 1. Read the TZ environment variable.  If it is set, use it.
+        /// 2. Get the default TZ from the device
+        /// 3. Use UTC if all else fails.
+        ///
+        /// On all other platforms
         /// 1. Read the TZ environment variable.  If it is set, use it.
         /// 2. Look for the data in /etc/localtime.
         /// 3. Look for the data in GetTimeZoneDirectory()/localtime.
@@ -393,16 +401,21 @@ private static bool TryGetLocalTzFile([NotNullWhen(true)] out byte[]? rawData, [
             id = null;
             string? tzVariable = GetTzEnvironmentVariable();
 
-            // If the env var is null, use the localtime file
+            // If the env var is null, on iOS/tvOS, grab the default tz from the device.
+            // On all other platforms, use the localtime file.
             if (tzVariable == null)
             {
+#if TARGET_IOS || TARGET_TVOS
+                tzVariable = Interop.Sys.GetDefaultTimeZone();
+#else
                 return
                     TryLoadTzFile("/etc/localtime", ref rawData, ref id) ||
                     TryLoadTzFile(Path.Combine(GetTimeZoneDirectory(), "localtime"), ref rawData, ref id);
+#endif
             }
 
             // If it's empty, use UTC (TryGetLocalTzFile() should return false).
-            if (tzVariable.Length == 0)
+            if (string.IsNullOrEmpty(tzVariable))
             {
                 return false;
             }

src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs

@@ -2887,6 +2887,13 @@ public static void NoBackwardTimeZones()
             Assert.Equal(tzCollection.Count, tzDisplayNames.Count);
         }
 
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Android | TestPlatforms.iOS | TestPlatforms.tvOS)]
+        public static void LocalTzIsNotUtc()
+        {
+            Assert.NotEqual(TimeZoneInfo.Utc.StandardName, TimeZoneInfo.Local.StandardName);
+        }
+
         private static bool IsEnglishUILanguage => CultureInfo.CurrentUICulture.Name.Length == 0 || CultureInfo.CurrentUICulture.TwoLetterISOLanguageName == "en";
 
         private static bool IsEnglishUILanguageAndRemoteExecutorSupported => IsEnglishUILanguage && RemoteExecutor.IsSupported;