SYNTAX_ERROR: line 6:19: Unexpected parameters (timestamp, integer, integer) for function substr. Expected: substr(varchar(x), bigint, bigint)

SELECT distinct substr(ORDER_DATE,1,10) mm, pzd.PART_KEY part_key FROM PZDTORBK PZD WHERE PZD.ORDER_DATE > TO_DATE ('2021-01-01','YYYY-MM-DD') --AND (substr(PO_NUMBER,5,1) <> 'L' and substr(PO_NUMBER,-2,2) <> '02') --group by to_char(PZD.ORDER_DATE,'MON'