Fenrisulfr
Crowing
The day you set is day zero, and every 24 hrs that passes is a day. So if you set Tuesday at 6pm that's "day zero", and Wednesday at 6pm officially competes day one because they eggs have officially lived in the incubator for 24 hrs, or one complete day. Lockdown will begin After 336 hours, or 14 compete days in the incubator.
My understanding is early hatches mean the incubator was too hot, late hatches mean the incubator was too cold.
As long as the whole batch doesn't hatch on day 16, it is just normal, when a few early hatchers or laties occure.
Nature is no digital machine.