code: public static String dateSub(String dateStr, int days, TimeZone tz) {
  		long ts = parseToTimeMillis(dateStr, tz);
  		if (ts == Long.MIN_VALUE) {
  			return null;
  		}
  		return dateSub(ts, days, tz);
  	}
docs: Do subtraction on date string.
  
  @param dateStr formatted date string.
  @param days days count you want to subtract.
  @param tz time zone of the date time string
  @return datetime string.