If you connect via a router, go into the setup there and see if it tracks the statistics you want (probably on the WAN connection). I know my own router (which has been flashed to dd-wrt) keeps track of these numbers - I can flip through charts all the way back to when I installed this particular router on 07/14/2010 and see how much I uploaded and downloaded on any particular day, and also see aggregate totals for each month 08/2010-present.