docs: public static String format(long duration, TimeUnit unit, boolean trimLeftZeros){
  		String format;
  		switch (unit){
  		case HOURS:
  			format = "HH";
  			break;
  		case MINUTES:
  			format = "HH:mm";
  			break;
  		case SECONDS:
  			format = "HH:mm:ss";
  			break;
  		case MILLISECONDS:
  			format = "HH:mm:ss.SSS";
  			break;
  		default:
  			throw new IllegalArgumentException("The unit must be HOURS, MINUTES, SECONDS, or MILLISECONDS.");
  		}
  		
  		DateFormat df = new SimpleDateFormat(format);
  		df.setTimeZone(TimeZone.getTimeZone("GMT"));
  		String resultWithZeros;
  		if (duration >= ONE_DAY_IN_MILLI){
  			long days = duration/ONE_DAY_IN_MILLI;
  			resultWithZeros = String.format("%dd, %s", days, df.format(duration));
  		}else{
  			resultWithZeros = df.format(duration);
  		}
  		return trimLeftZeros ? 
  				resultWithZeros.replaceFirst("^[0 :]+", "") : resultWithZeros;
  	}