Yes it is approx 3 weeks after a broody starts actively sitting on the eggs where she is on them day and night except to occasionally get off to eat, drink and poop. If you are incubating it starts the day you put the eggs in the incubator or when a broody starts sitting. The heat starts the development. How you count is your preference. It depends if you count the day you set your eggs as day 0 or day 1. I start out the day I set the eggs as day 0. I think of it like this, you aren't 1 yr old the day you are born the same I think the same should apply to hatching. I don't count the day I set the eggs for hatching. I start the following day as day 1. That is just the way I do it.