Oracle 18c

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CREATE TABLE table_name (
  start_time DATE,
  sla        INTERVAL DAY TO SECOND
);
INSERT INTO table_name ( start_time, sla ) VALUES (
  TO_DATE('01-Jun-2021 19:15:38', 'DD-MON-YYYY HH24:MI:SS', 'NLS_DATE_LANGUAGE=American'),
  INTERVAL '20:10:00' HOUR TO SECOND
);
1 rows affected

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT start_time,
       sla,
       (start_time - TRUNC(start_time)) DAY TO SECOND - sla AS difference
FROM   table_name


START_TIME
SLA
DIFFERENCE


2021-06-01 19:15:38
+00 20:10:00.000000
-000000000 00:54:22.000000000


SELECT start_time,
       sla,
       CASE WHEN difference < INTERVAL '0' HOUR THEN '-' END
       || TO_CHAR( ABS( EXTRACT( HOUR FROM difference ) ), 'FM00' )
       || TO_CHAR( ABS( EXTRACT( MINUTE FROM difference ) ), 'FM00' )
         AS difference
FROM   (
  SELECT start_time,
         sla,
         (start_time - TRUNC(start_time)) DAY TO SECOND - sla AS difference
  FROM   table_name
)


START_TIME
SLA
DIFFERENCE


2021-06-01 19:15:38
+00 20:10:00.000000
-0054