it has 2 counters, default is for the current trip which resets every time you stop for more than about 2 hours. Second one is total since new.
To access the second Trip you need to select it with the reset button under the wiper arm. I do not have manual to hand, but i believe its press and hold until it flips over. It will then stay on that one until moved back. Check the Manual, its in there somewhere.