Forms :: FRM-40735 When Button Pressed Trigger Raised Unhandled Exception ORA-01843

I am trying to insert some values into a table from a form i get the below error;

which says not a valid month;

FRM-40735 WHEN-BUTTON-PRESSED trigger raised unhandled exception ORA-01843

after i do google i added the below one to the insert statement and again i receive the error;

to_date(to_char(sysdate,'dd/mm/yyyy'),'dd/mm/yyyy')

My Insert Statement

insert into Invoice123
(Invoiceno, invdate, gross_tot, discount)
values
(nvl(INVOICEno(j),0),
to_date(to_char(invdate(j),'dd/mm/yyyy'),'dd/mm/yyyy'));
commit;

by which format we can achieve this!