code: public <UNIT extends TimeUnit> TimeFormat removeUnit(final Class<UNIT> unitType)
     {
        if (unitType == null)
           return null;
  
        for (TimeUnit unit : units.keySet()) {
           if (unitType.isAssignableFrom(unit.getClass()))
           {
              cachedUnits = null;
  
              return units.remove(unit);
           }
        }
        return null;
     }
docs: Removes the mapping for the given {@link TimeUnit} type. This effectively de-registers the {@link TimeUnit} so it
  will not be used in formatting. Returns the {@link TimeFormat} that was removed, or <code>null</code> if no unit
  of the given type was registered.