The way to correct it is to change the clock frequency of the clock. That involves a little bit of engineering know-how. (It's probably not adjustable, so you'd have to hand-pick parts to adjust it, probably with a twisted wire capacitor for fine adjustment. That should get it down to as close as whatever you're using to measure it with. A really good frequency counter, clocked with WWVL or GPS signals, could probably get you to a few seconds a month.)
Or you can just reset it every week.
srkmagnus, Automatic time is set from the cell towers, which a wifi-only tab can't do.