It all depends on server downtime really. Since it's 8 days exactly, you just have to go back an decide how much more time to add due to that extra time. You can convince yourself of this by timing items that spawn every 30 minutes (skull candle). You can set your watch by it. Well, at least you used to be able to. It's been a while since I played with my doom looter, but I actually have a sub that computes the next spawn time to the day/hour.