On a very long cross country the Hobbs time didn't seem to agree with my watch or GNS430 timer. While in cruise flight, the elapsed time (about two hours) of the AF-2500 is off by about 12-13%.....it shows less time than actual. It also seems to be a bit variable.....about 12% for one test, about 13% for another test on another day. Does AFS know what would cause this, if it could be variable, if it likely existed since the unit was new? I have used this timer for all log book entries (flight and maintenance) over the last 1100 hours or so......not good.