Will the chicks hatch based on when they were layed or when the hen starts sitting on them?

[chick peeps]

Sorry the title was so long. I just got some hatching eggs a week ago and I don't know that exact day them were born. I was wondering if they would hatch based on how long the hen has been sitting on them or based on what day they were layed? I would really like to know!
- chickpeeps

 

[Ol Grey Mare]

Chicks don't start to develop until incubation begins (either the hen starts setting or you put them in the incubator) - so they hatch based on the day you start incubation, not the day they are laid.

 

[chick peeps]

Thanks! Then I have it all planned right!