--- old/src/java.base/share/classes/java/time/Clock.java	2019-08-27 17:28:25.994324386 -0700
+++ new/src/java.base/share/classes/java/time/Clock.java	2019-08-27 17:28:25.814324386 -0700
@@ -479,6 +479,7 @@
      * {@link System#currentTimeMillis()}.
      */
     static final class SystemClock extends Clock implements Serializable {
+        @java.io.Serial
         private static final long serialVersionUID = 6740630888130243051L;
         private static final long OFFSET_SEED =
                 System.currentTimeMillis()/1000 - 1024; // initial offest
@@ -572,6 +573,7 @@
         public String toString() {
             return "SystemClock[" + zone + "]";
         }
+        @java.io.Serial
         private void readObject(ObjectInputStream is)
                 throws IOException, ClassNotFoundException {
             // ensure that offset is initialized
@@ -586,6 +588,7 @@
      * This is typically used for testing.
      */
     static final class FixedClock extends Clock implements Serializable {
+        @java.io.Serial
         private static final long serialVersionUID = 7430389292664866958L;
         private final Instant instant;
         private final ZoneId zone;
@@ -636,6 +639,7 @@
      * Implementation of a clock that adds an offset to an underlying clock.
      */
     static final class OffsetClock extends Clock implements Serializable {
+        @java.io.Serial
         private static final long serialVersionUID = 2007484719125426256L;
         private final Clock baseClock;
         private final Duration offset;
@@ -686,6 +690,7 @@
      * Implementation of a clock that adds an offset to an underlying clock.
      */
     static final class TickClock extends Clock implements Serializable {
+        @java.io.Serial
         private static final long serialVersionUID = 6504659149906368850L;
         private final Clock baseClock;
         private final long tickNanos;