code: public static String formatDuration(long millis) {
  		StringBuilder sb = new StringBuilder();
  		if(millis > DAY_MILLIS) {
  			sb.append(millis / DAY_MILLIS);
  			sb.append(" days, ");
  			millis %= DAY_MILLIS;
  		}
  		if(millis > HOUR_MILLIS) {
  			sb.append(millis / HOUR_MILLIS);
  			sb.append(" hours, ");
  			millis %= HOUR_MILLIS;
  		}
  		if(millis > MINUTE_MILLIS) {
  			sb.append(millis / MINUTE_MILLIS);
  			sb.append(" minutes, ");
  			millis %= MINUTE_MILLIS;
  		}
  		if(millis > SECOND_MILLIS) {
  			sb.append(millis / SECOND_MILLIS);
  			sb.append(" seconds, ");
  			millis %= SECOND_MILLIS;
  		}
  		sb.append(millis);
  		sb.append(" milliseconds");
  		return sb.toString();
  	}
docs: Formats a duration in milliseconds as
  a pretty-print string.
  
  @param millis Duration in milliseconds.
  @return Given duration as a pretty-print string.