code: public static final Map<String, DateTimeZone> getDefaultTimeZoneNames() {
          Map<String, DateTimeZone> names = cZoneNames.get();
          if (names == null) {
              names = buildDefaultTimeZoneNames();
              if (!cZoneNames.compareAndSet(null, names)) {
                  names = cZoneNames.get();
              }
          }
          return names;
      }
docs: Gets the default map of time zone names.
  <p>
  This can be changed by {@link #setDefaultTimeZoneNames}.
  <p>
  The default set of short time zone names is as follows:
  <ul>
  <li>UT - UTC
  <li>UTC - UTC
  <li>GMT - UTC
  <li>EST - America/New_York
  <li>EDT - America/New_York
  <li>CST - America/Chicago
  <li>CDT - America/Chicago
  <li>MST - America/Denver
  <li>MDT - America/Denver
  <li>PST - America/Los_Angeles
  <li>PDT - America/Los_Angeles
  </ul>
  
  @return the unmodifiable map of abbreviations to zones, not null
  @since 2.2