code: public PrettyTime registerUnit(final TimeUnit unit, TimeFormat format)
     {
        if (unit == null)
           throw new IllegalArgumentException("Unit to register must not be null.");
        if (format == null)
           throw new IllegalArgumentException("Format to register must not be null.");
  
        cachedUnits = null;
  
        units.put(unit, format);
        if (unit instanceof LocaleAware)
           ((LocaleAware<?>) unit).setLocale(locale);
        if (format instanceof LocaleAware)
           ((LocaleAware<?>) format).setLocale(locale);
        return this;
     }
docs: Register the given {@link TimeUnit} and corresponding {@link TimeFormat} instance to be used in calculations. If
  an entry already exists for the given {@link TimeUnit}, its {@link TimeFormat} will be overwritten with the given
  {@link TimeFormat}. ({@link TimeUnit} and {@link TimeFormat} must not be <code>null</code>.)