I finally thought about it in hours. 1 day is 24hrs. 24hrs from when I set them would then be 1 day. After 504 hrs or 21 days they are due to hatch. So if I set them 10pm tonight they are due 10 pm on the 8th. However eggs rarely start hatching at exactly 21 days. They could start hatching from day 18-22 and still be just fine. Mine have been pretty accurate so far. Aside from a couple early pips last hatch they have pipped late on day 20 and started hatching day 21.