[Sienna Wheeler]

I have 13 duck eggs in an incubator downstairs and we predicted that they are on day 23 right now, however we are unsure as to wether that is correct and, after candling the eggs, I think they may be slightly older. I have decided to move the hatching mat into the incubator in a few days, but don't want to be too late. How can I tell when they are about to hatch / when is the earliest I can move the hatching mat in?