Bank of hours (balance)
Record a bank of hours - I work 8 hours a day; when I work less than it, I get a debt in this bank of hours; when I work more than it, I get a credit in the bank. So I can control how many hours I have in balance and even take a day off or more if it sums to 8 or more credit hours. It should count only billable time records. And it should have an option to clear current balance (for instance, if I get paid for it).