code: public static long firstOfLastMonthTime(final long unixTimestamp) {
          final Calendar cal = createFirstOfMonthCal(unixTimestamp);
          cal.add(Calendar.MONTH, -1);
  
          return cal.getTimeInMillis() / 1000L;
      }
docs: @param unixTimestamp Current time
  
  @return Timestamp for the beginning of the previous month relative to current time