core: Vastly separate types of clock in FakeClock

There was an attempt to use different epochs for the wall clock and the
monotonic clock. However, 123456789 is actually less than a second.
We want the gap between clocks to be at least a day. This issue was
discovered in #8968.

This separation found a bug in an RLS test where it was mixing epochs.
However, it was only a problem in the test. The code under test is
wrongly using wall clock for calculation durations, but that seems to be
a wide-spread problem and will need to be handled separately.

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

@@ -357,7 +357,7 @@ public final class FakeClock {
 
   public long currentTimeMillis() {
     // Normally millis and nanos are of different epochs. Add an offset to simulate that.
-    return TimeUnit.NANOSECONDS.toMillis(currentTimeNanos + 123456789L);
+    return TimeUnit.NANOSECONDS.toMillis(currentTimeNanos + 1234567890123456789L);
   }
 
   /**

rls/src/test/java/io/grpc/rls/AdaptiveThrottlerTest.java

@@ -41,6 +41,8 @@ public class AdaptiveThrottlerTest {
 
   @Test
   public void shouldThrottle() {
+    long startTime = fakeClock.currentTimeMillis();
+
     // initial states
     assertThat(throttler.requestStat.get(fakeTimeProvider.currentTimeNanos())).isEqualTo(0L);
     assertThat(throttler.throttledStat.get(fakeTimeProvider.currentTimeNanos())).isEqualTo(0L);
@@ -71,8 +73,8 @@ public class AdaptiveThrottlerTest {
         .isWithin(TOLERANCE)
         .of(1.0f / 3.0f);
 
-    // Skip half a second (half the duration).
+    // Skip to half second mark from the beginning (half the duration).
-    fakeClock.forwardTime(500 - fakeClock.currentTimeMillis(), TimeUnit.MILLISECONDS);
+    fakeClock.forwardTime(500 - (fakeClock.currentTimeMillis() - startTime), TimeUnit.MILLISECONDS);
 
     // Request 3, throttled by backend
     assertThat(throttler.shouldThrottle(0.4f)).isFalse();
@@ -96,7 +98,8 @@ public class AdaptiveThrottlerTest {
         .of(3.0f / 5.0f);
 
     // Skip to the point where only requests 3 and 4 are visible.
-    fakeClock.forwardTime(1250 - fakeClock.currentTimeMillis(), TimeUnit.MILLISECONDS);
+    fakeClock.forwardTime(
+        1250 - (fakeClock.currentTimeMillis() - startTime), TimeUnit.MILLISECONDS);
 
     assertThat(throttler.requestStat.get(fakeTimeProvider.currentTimeNanos())).isEqualTo(2L);
     assertThat(throttler.throttledStat.get(fakeTimeProvider.currentTimeNanos())).isEqualTo(2L);