Oracle 18c

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CREATE TABLE Appointment( ApptDate VARCHAR2(15), ApptTime VARCHAR2(15) )
INSERT INTO Appointment
SELECT '9/26/21','9:00 AM' FROM dual UNION ALL
SELECT '9/25/20','1:00 PM' FROM dual
2 rows affected

CREATE TABLE Appointment2 AS
SELECT TO_DATE(ApptDate||' '||ApptTime,'MM/DD/RR HH:MI PM') AS ApptDate      
  FROM Appointment
2 rows affected

DROP TABLE Appointment
RENAME Appointment2 TO Appointment
SELECT TO_CHAR(ApptDate,'RRRR-MM-DD HH:MI PM') AS ApptDate FROM Appointment


APPTDATE


2021-09-26 09:00 AM

2020-09-25 01:00 PM


--- For the second case
DROP TABLE Appointment
CREATE TABLE Appointment( ApptDate DATE, ApptTime VARCHAR2(15) )
INSERT INTO Appointment
SELECT date'2021-09-26','9:00 AM' FROM dual UNION ALL
SELECT date'2020-09-25','1:00 PM' FROM dual
2 rows affected

CREATE TABLE Appointment2 AS
SELECT TO_DATE(ApptDate||' '||ApptTime,'RRRR-MM-DD HH:MI PM') AS ApptDate      
  FROM Appointment
2 rows affected

DROP TABLE Appointment
RENAME Appointment2 TO Appointment
SELECT TO_CHAR(ApptDate,'RRRR-MM-DD HH:MI PM') AS ApptDate FROM Appointment


APPTDATE


2026-09-21 09:00 AM

2025-09-20 01:00 PM