I count it from the first hour I set them. The three hatches I've had so far if I counted it after the first twenty four hours they all came early. Now I count it from the moment I put them in the box.
All of mine hatched on day 19 strong and healthy. It's over 100 here so maybe that's why (mine were under a broody) Maybe your temp was a little high and that's why they're hatching sooner?