SD card module. Maybe something like this. For logging over such a long period a battery backed real time clock module might also be useful.
http://www.ebay.co.uk/itm/DS3231-AT24C32-IIC-module-precision-Real-time-clock-quare-memory-for-ArduinoT5-/400679939769?pt=UK_Computing_Laptops_EH&hash=item5d4a62aeb9