core: combined epoch seconds and nanos changes in TimeProvider

core/src/main/java/io/grpc/internal/TimeProvider.java

@@ -16,7 +16,7 @@
 
 package io.grpc.internal;
 
-import java.util.concurrent.TimeUnit;
+import java.time.Instant;
 
 /**
  * Time source representing the current system time in nanos. Used to inject a fake clock
@@ -29,7 +29,10 @@ public interface TimeProvider {
   TimeProvider SYSTEM_TIME_PROVIDER = new TimeProvider() {
     @Override
     public long currentTimeNanos() {
-      return TimeUnit.MILLISECONDS.toNanos(System.currentTimeMillis());
+      Instant instant = Instant.now();
+      int nanos = instant.getNano();
+      long epochSeconds = instant.getEpochSecond();
+      return epochSeconds * 1_000_000_000L + nanos;
     }
   };
 }

core/src/test/java/io/grpc/internal/TimeProviderTest.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2024 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc.internal;
+
+import static org.junit.Assert.assertTrue;
+
+import java.time.Instant;
+import org.junit.Test;
+
+/**
+ *  Unit tests for {@link TimeProvider}.
+ */
+public class TimeProviderTest {
+  @Test
+  public void testCurrentTimeNanos() {
+
+    // Get the current time from the TimeProvider
+    long actualTimeNanos = TimeProvider.SYSTEM_TIME_PROVIDER.currentTimeNanos();
+
+    // Get the current time from Instant for comparison
+    Instant instantNow = Instant.now();
+    long expectedTimeNanos = instantNow.getEpochSecond() * 1_000_000_000L + instantNow.getNano();
+
+    // Validate the time returned is close to the expected value within a tolerance.
+    long toleranceOfTenMilliSecond = 10_000_000L; // 10 millisecond tolerance in nanoseconds
+    assertTrue("The current time in nanoseconds should be close to the expected time.",
+        Math.abs(actualTimeNanos - expectedTimeNanos) < toleranceOfTenMilliSecond);
+  }
+}