Throw when trying to set creation time of a DeterministicKey that is a leaf in the hierarchy. It would be needlessly stored, but unreadable.

core/src/main/java/org/bitcoinj/crypto/DeterministicKey.java

@@ -441,6 +441,18 @@ public class DeterministicKey extends ECKey {
             return super.getCreationTimeSeconds();
     }
 
+    /**
+     * The creation time of a deterministic key is equal to that of its parent, unless this key is the root of a tree.
+     * Thus, setting the creation time on a leaf is forbidden.
+     */
+    @Override
+    public void setCreationTimeSeconds(long newCreationTimeSeconds) {
+        if (parent != null)
+            throw new IllegalStateException("Creation time can only be set on root keys.");
+        else
+            super.setCreationTimeSeconds(newCreationTimeSeconds);
+    }
+
     /**
      * Verifies equality of all fields but NOT the parent pointer (thus the same key derived in two separate heirarchy
      * objects will equal each other.

core/src/test/java/org/bitcoinj/crypto/ChildKeyDerivationTest.java

@@ -199,7 +199,6 @@ public class ChildKeyDerivationTest {
 
         // Creation time can't survive the xpub serialization format unfortunately.
         key1.setCreationTimeSeconds(0);
-        key2.setCreationTimeSeconds(0);
         NetworkParameters params = MainNetParams.get();
 
         {