Repairing - only 88688 days to go!

I’m upgrading a DS218+ with a pair of new 20 tb drives. Upgrading from 8 tb. First swap went well, it did the fast repair thing overnight. 2nd swap now shows 88688 days (and 5 hrs 21 min, but who’s counting?) and hasn’t budged in a while. I read that it can take a while but that seems like rather a long time. Is this a sign of a bad drive, am I just being impatient, am I doing something wrong? Help pls.