Move thread block into CompareExchange

src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.Android.cs

@@ -32,25 +32,24 @@ public static DateTimeOffset Now
                     return ToLocalTime(utcDateTime, true);
                 }
 
-                if (Interlocked.CompareExchange(ref s_androidTZDataLoaded, 0, -1) != -1)
+                // The cache isn't loaded yet.
+                if (Interlocked.CompareExchange(ref s_androidTZDataLoaded, 0, -1) == -1)
                 {
-                    return ToLocalTime(utcDateTime, true);
-                }
-
-                new Thread(() =>
-                {
-                    try
+                    new Thread(() =>
                     {
-                        // Delay the background thread to avoid impacting startup, if it still coincides after 1s, startup is already perceived as slow
-                        Thread.Sleep(1000);
+                        try
+                        {
+                            // Delay the background thread to avoid impacting startup, if it still coincides after 1s, startup is already perceived as slow
+                            Thread.Sleep(1000);
 
-                        _ = TimeZoneInfo.Local; // Load AndroidTZData
-                    }
-                    finally
-                    {
-                        Volatile.Write(ref s_androidTZDataLoaded, 1);
-                    }
-                }) { IsBackground = true }.Start();
+                            _ = TimeZoneInfo.Local; // Load AndroidTZData
+                        }
+                        finally
+                        {
+                            Volatile.Write(ref s_androidTZDataLoaded, 1);
+                        }
+                    }) { IsBackground = true }.Start();
+                }
 
                 object? localDateTimeOffset = AppContext.GetData("System.TimeZoneInfo.LocalDateTimeOffset");
                 if (localDateTimeOffset == null) // If no offset property provided through monovm app context, default

src/tasks/AndroidAppBuilder/Templates/MonoRunner.java

@@ -24,6 +24,7 @@
 import java.io.OutputStream;
 import java.io.BufferedInputStream;
 import java.util.ArrayList;
+import java.util.Calendar;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
 import java.time.OffsetDateTime;
@@ -90,7 +91,7 @@ public static int initialize(String entryPointLibName, String[] args, Context co
         unzipAssets(context, filesDir, "assets.zip");
 
         Log.i("DOTNET", "MonoRunner initialize,, entryPointLibName=" + entryPointLibName);
-        int localDateTimeOffset = OffsetDateTime.now().getOffset().getTotalSeconds();
+        int localDateTimeOffset = getLocalDateTimeOffset();
         return initRuntime(filesDir, cacheDir, testResultsDir, entryPointLibName, args, localDateTimeOffset);
     }
 
@@ -152,6 +153,16 @@ static void unzipAssets(Context context, String toPath, String zipName) {
         }
     }
 
+    static int getLocalDateTimeOffset() {
+        if (android.os.Build.VERSION.SDK_INT >= 26) {
+            return OffsetDateTime.now().getOffset().getTotalSeconds();
+        }
+        else {
+            int offsetInMillis = Calendar.getInstance().getTimeZone().getRawOffset();
+            return offsetInMillis / 1000;
+        }
+    }
+
     static native int initRuntime(String libsDir, String cacheDir, String testResultsDir, String entryPointLibName, String[] args, int local_date_time_offset);
 
     static native int setEnv(String key, String value);