The time zone data file format has been stable since its release in 1.4.0. We've made an upward compatible change for fixing a problem of future GMT offset changes (5055567). Since the Java runtime still can read the file format and Sun can control time zone data file update, we didn't change the format version. However, it's necessary to change the version when we release public javazic tool in Mustang so that the new format is rejected by the runtime having no 5055567 fix.