Date range logic in PL/SQL loop

Databases
1

Hi guys. I have a PL/SQL procedure as below to use loops to control data purging.Each loop execute to purge old data for 10 days until the remaining records left is only 30 days. However, I notice this code will purge data for exactly 30 days ago (SYSDATE - 30) which should be maintained in database.


DECLARE
      days2keep      PLS_INTEGER    := 30;
      startDate       VARCHAR2 (50)     :=  SYSDATE - 70;
      sql_stmt        VARCHAR2 (300);
      i                   VARCHAR2 (50);

   BEGIN
   
   i :=  startDate ; 
   LOOP       
      sql_stmt :=
            'BEGIN INSERT INTO HIST_TBL '
         || ' (SELECT * FROM  TBL'
         || ' WHERE trandt BETWEEN '
         || i
         || 'AND' i + 10
         || '); DELETE FROM  TBL'
         || ' WHERE trandt BETWEEN '
         || i
         || 'AND' i + 10
         || '; end;';
    
      EXECUTE IMMEDIATE sql_stmt;

      i := i + 10;
      EXIT WHEN i >= SYSDATE - 30;
 END LOOP;

1st loop: SYSDATE - 70 –> SYSDATE - 60 (Purged)
2nd loop: SYSDATE - 60 –> SYSDATE - 50 (Purged)
3rd loop: SYSDATE - 50 –> SYSDATE - 40 (Purged)
4th loop: SYSDATE - 40 –> SYSDATE - 30 (SYSDATE-30 is purged as well, how can avoid it?)

2

Start with 71 ?