multi-field unique key

[vantis]

Is there any way to have two fields combined to create a unique record. I have a timesheet database. The fields I would like to have unique are employee_id and time_start_date. I d not need a many:many realtionship. I just need to make sure that no one employee can be associated to the same week more than once. Both fields are in the same database. Any suggestion on implemneting a combo key? I tried creating a calculation but I am not sure if you can apply uniqueness to the calc.