postgresql – Converting DATE and TIME Ingeters to Timestamp in postgres

I am trying to get a Timestamp by combining Date and Time columns in my PostgresQL database and am running into errors. Here is what I have.

+-----------+-----------+
| StartDate | StartTime |
+-----------+-----------+
| 20210301  | 18000000  |
+-----------+-----------+
| 20210302  | 19000000  |
+-----------+-----------+
| 20210303  | 20000000  |
+-----------+-----------+

Here is what I want

+-----------+-----------+-----------------------+
| StartDate | StartTime | StartStamp            |
+-----------+-----------+-----------------------+
| 20210301  | 18000000  | 2021-03-01 18:00:00.0 |
+-----------+-----------+-----------------------+
| 20210302  | 19000000  | 2021-03-02 19:00:00.0 |
+-----------+-----------+-----------------------+
| 20210303  | 20000000  | 2021-03-03 20:00:00.0 |
+-----------+-----------+-----------------------+

In MySQL I would be able to get this by using the following sql

SELECT  STR_TO_DATE(concat(LPAD("STARTDT", 8, '0'),'-', LEFT(LPAD("STARTTM", 8, '0'),6)),'%Y%m%d-%H%i%s') AS StartStamp
From Table1 

To get that in postgres, I’ve tried to CAST(STARTDT as DATE) but that resulted in an error. I’ve tried TO_DATE() but that also did not work. Is there a way to get the timestamp in PostgresQL ?