public static long getFirstDay4Month(int year, int month) {
        long firstDay = 0L;
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.MONTH, month - 1);
        cal.set(Calendar.DAY_OF_MONTH, 1);
        cal.set(Calendar.HOUR_OF_DAY, 0);
        cal.set(Calendar.MINUTE, 0);
        cal.set(Calendar.SECOND, 0);
        cal.set(Calendar.MILLISECOND, 0);
        firstDay = cal.getTimeInMillis() / 1000L;
        return firstDay;
    }
@author sishu.yss
@param year
@param month
@return
core/src/main/java/com/dtstack/flink/sql/util/DateUtil.java