Using Date Parameters in SQL*Plus report (as Concurrent Program)

 Im trying to write a SQL*Query as a report to install in the concurrent manager.
The report contains date parameters and I am using a Standard Date validation set.  
However because each user can use different date formats when they submit the report I cannot use a standard to_date in the SQL.

Is there an easy way to convert to a date when you don't know what the format being passed in is?

and c.check_date between to_date('&&STARTDATE','DD-MON-YYYY')
                                    and to_date('&&ENDDATE','DD-MON-YYYY')


Thanks,
Mary