Conditionally counting records based on field on new record

You would need to pass timesheet_date down to the timesheet_employees table. 

then, do a summary field between employees and timesheet_employees, counting the number of records where timesheet_date = today and per diem is true. Then, pass that summary back down to the timesheet_employees table as a lookup field. 

In your form rule, you would do something like: if [employees - # of per diems today] > 0 hide [per diem]. 

That should work. 

Let me know if you run into trouble.

Thanks,

Eric