code: public static String dateSub(long ts, int days, TimeZone tz) {
  		ZoneId zoneId = tz.toZoneId();
  		Instant instant = Instant.ofEpochMilli(ts);
  		ZonedDateTime zdt = ZonedDateTime.ofInstant(instant, zoneId);
  		long resultTs = zdt.minusDays(days).toInstant().toEpochMilli();
  		return dateFormat(resultTs, DATE_FORMAT_STRING, tz);
  	}
docs: Do subtraction on date string.
  
  @param ts    the timestamp.
  @param days days count you want to subtract.
  @param tz time zone of the date time string
  @return datetime string.