I agree with Debbie. A few hours early should not matter. Mother Nature may be pretty accurate but she is really not that precise.
Below is something I wrote up about it for another post. For some people, what is considered Day 1 or Day 18 can be confusing. Maybe this will help you remember.
There is a simple way to look at when the eggs should hatch. If you start the eggs on a Wednesday, they should hatch on a Wednesday. If you set them on a Sunday, they should hatch on a Sunday. Whatever day of the week you set them will be the day they should hatch. Some may hatch early. Some may hatch late. Some may even hatch when they are supposed to. Mine were a day and a half early, which means the incubation temperature was a little too high.
Hatching does not occur instantaneously but is a process. They pip. They rest, sometimes a long time. They zip. They push out of the shell. They rest and dry off. It is an exhausting process for the tiny little fellow.
Chicken eggs take about 21 days to develop and hatch. Some are a bit faster or slower but about 21 days. The eggs have not had 24 hours of development the instant you put them in the incubator. It takes 24 hours to get 24 hours of development. If you have a photo that says, when you candle at day 10, the development should look like this, that is after 10 full days of development. It is not after 9 days and 1 second of development. If you are looking at hatch day, it is at the end of 21 full days of development.
Lockdown should be after 18 days of development. A few hours early is not all that critical.
