Subform event to update automaticaly Form

Hi

I have a form and a subform



Which method should I use if I need to update automaticaly a field in form based on an event in subform ?



Just to be more clear, have a look in the below example.



In first print screen, student "Johnson Nick" has not completed all tests in TestA subform, the form field "DateCompleteA" is blanc.



Once a student completes all tests in Period A, I need the form field DateCompleteA to show the date that he completed the last one. (18/3/2015).

If student has not yet completed all tests in TestA subform the Form field DateCompleteA must be blanc.



Student has not completed "literature"yet, DateCompletA field is blanc





Student completed ALL his test, subform is updated, Form field DateCompletA is updated with the maximum date

Subform event to update automaticaly Form