Lockdown for chicken eggs is after 18 days of development. Some people get confused about that. An egg does not have one day of development one second after it goes in the incuubator. It has one day of development after 24 hours or one full day of development.
If you put your eggs in the incubator at 7:00 p.m. on Monday May 3, they have had one day of development at 7:00 p.m. on Tuesday May 4. Lockdown would be after 18 days of development or Friday, May 21 at 7:00 p.m. It is not an exact science. A few hours either way won't make any real difference, but that is the general target. In this example, 21 days of development is completed on Monday, May 24 at 7:00 p.m.
Hatching is not instantaneous. A egg pips. The chick rests, sometimes a long time. The egg zips. After a while, the chick pushes the parts of the shell apart. It is wet and very tired. It rests. But after a while it is running all over the incubator, knocking around the other eggs. Some hatch earlier or later than others. If your average incubating temperature is a bit high, the eggs can hatch a day or more early. If the temperatures are a bit low, they can be a day or two late.
Hope that helps.