private CachedDeserializable getCd() {
      if (event != null && !event.offHeapOk) {
        throw new IllegalStateException(
            "Attempt to access off heap value after the EntryEvent was released.");
      }
      return cd;
    }
this method joins together version tag timestamps and the "lastModified" timestamps generated
and stored in entries. If a change does not already carry a lastModified timestamp

@return the timestamp to store in the entry
geode-core/src/main/java/org/apache/geode/internal/cache/EntryEventImpl.java