OK, I'm assuming you're trying something like
select column1
from table1
where date_col > '01-may-2003 14:00:00'
and it is giving you an ORA-01830 error.
If this is the case then you need to alter your session so that Oracle can recognise the appropriate string as a valid date format.
You can use
ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY HH24:MI:SS';
where the string is the format of the date/time format that you want to use.
If this isn't the problem can you give a few more details.