This is a bit of a guess but your problem is similar to one I had.
I had a digital timer, like a stop watch but it only counted whole seconds. I put a new battery in after a long time of not using it. I then found it was not counting every fourth second, so it counted 5 seconds as 4.
so it would count
0:00 <1 second gap> 0:01 <1 sec gap> 0:02 <1 sec gap> 0:03 <2 sec gap> 0:04 <1 sec gap> 0:05 <1 sec gap> 0:06 <1 sec gap> 0:07 <2 sec gap> 0:08 <1 sec gap> 0:09 <1 sec gap> etc....
So the amount the clock is slow by is one fifth of the total time elapsed. You say it was slow by 17 hours, 5*17 = 85 hours = 3 days 13 hours. Which sounds like the "few days" you left it for!
I managed to fix my timer simply by turning it off and on again. So maybe somehow temporarily breaking the connection between the battery and the a3000 would reset the hardware clock and get it counting properly again.
