How long does it take for chick development to start?

[Pufna]

So I started collecting eggs for my broody hen, and suddenly another one became broody and stayed on the favorite nest for more than 12 hours (around 15h, but I'm not really sure if it was a bit more or less).
Now I'm wondering if I can place those eggs that were under a broody hen for like 15h with fresh ones, will they hatch earlier or will they hatch at all now that they cooled off?
And how long does it take for the development to start?

 

[nuthatched]

Development takes about a day, they may hatch a little earlier but not by much.

 

[SilverBirds]

Development starts on day one. If you crack a fertile egg open that has been incubating for one day you will see red veins already covering the yolk. Not yet visible from the outside of the egg but still there.