Please help! Paid Time Off...

Greetings and good morning...
I am trying to set up a form with an accrual system. I have the following fields established in my form - date, employee (look up field), PTO, and start date. What I'm trying to do is have the start date remain a constant so I don't have to fill it in every time I record someone's absence (the start time plays into the accrual formula). Also, how do I update an employee's balance once I record days off. The records are unrelated...what is the method (script, function, etc), to update the employee's balance every time I submit a record?

Thanks! Your help is greatly appreciated.