I want my employees to enter their daily working hours onto a form.  
Each period of work should span over 4 weeks and show the hours they are contracted to do each day and the hours they have actually worked.  Then a +/- balance is which is calculated and shown in a weekly balance field.  
Each period will have a period number and a corresponding commencement date. I want to be able to pull off staff hours balances by the period number.
The staff are also contracted to spend 60% of their working hours in the office as opposed to home working and a calculated field showing that percentage.
I have a Timesheet Table, a Staff Table and a Date Table (showing the period number and commencement dates.
Question: I'm a bit stuck on how to make sure that each period number does not allow dates outside that period to be used.  I'm guessing the commencement date and number should be linked.  Should the period number and commencement date be a composite key or am I barking up the wrong tree?