static long nextMonthDay(int index){
        long start=startDay.getTime();
        start+=index*milliSecondsOnyDay;
        Calendar calendar=new GregorianCalendar(TimeZone.getTimeZone("GMT"));
        calendar.setTime(new Date(start));
        int thisMonthDay=calendar.get(Calendar.DAY_OF_MONTH);
        calendar.add(Calendar.MONTH,1);
        int nextMonthDay=calendar.get(Calendar.DAY_OF_MONTH);
        if(thisMonthDay!=nextMonthDay){
            return -1;
        }
        return index+ (calendar.getTime().getTime()-start)/milliSecondsOnyDay;
    }