In Informix and SQL Server, you can use the DAY function to get the day (an integer number) of the specified datetime.
Informix:
-- Get the month SELECT DAY(TO_DATE('2024-12-06', '%Y-%m-%d')) FROM systables WHERE tabid=1; /* 6 */
SQL Server:
-- Get the day SELECT DAY('2024-12-06'); /* 6 */
For more information, see SQL Server to PostgreSQL Migration