Changes

Based on the fees paid, we can determine the validity of a patent. Each fee is coded and the DaysIncrement variable indicates how many more days a patent is valid for after that fee is paid. The DaysDuration is how many total days the patent has been valid for since it's grant date. DaysDuration is the key variable, for each fee paid, we add DaysIncrement to DaysDuration. The DaysDuration past the grant date is the validity of the patent.