I have a similar problem with my Maestro 3225. It is off by 3 minutes and 39 seconds right now and seems to be growing about 1 minute per week. Eventually the ETA will be useless without some extra math. A GPS receiver only works by measuring extremely precise timing signals, so I am sure that it internally knows time to better than a microsecond (probably nanoseconds) to tell me where I am as accurately as it does. Go figure.