Artemis- Recalling Eight Year Old Passcode

The Fowl Bentley was protected by a fingerprint scanner, and a keypad that required an eight-digit code. The code was changed every month, and so it took Artemis a few seconds to mentally rewind almost eight years and remember the right set of numbers.
He slid across the front seat’s tan leather upholstery and pressed his thumb to a second scanner tucked behind the steering wheel. A spring-loaded compartment slid smoothly from the dash. It was not a large compartment, but big enough to hold a clip of cash, platinum credit cards, and a spare cell phone in its cradle.

-The Time Paradox, Chapter 7