I agree with this.
Do you remember the day you actually put the eggs in the incubator and started them? What was the date and day of the week?
An egg does not have a day's worth of incubation the instant you put it in the incubator, it takes 24 hours for it to have a day's worth of incubation. So if you are counting the days you say "one" 24 hours after you put it in the incubator. So lets go through this. Say you put the eggs in the incubator on Thursday March 3rd. You say "one" on Friday March 4th. "two" Saturday March 5th, and so on. It actually helps to look at a calendar and touch the dates when you do this. If you do this you will say "18" on Monday, March 21st. That is lockdown. If you continue counting this way you will say "21" on Thursday March 24th.
An easy way to check your counting is that the day of the week you started them is the day of the week you should say "21". In my example starting them on Thursday the 3rd you say "21" on Thursday the 24th. From Thursday to Thursday. If you say "21" on the right day then you have to say "18" on the right day.
I'm not doing it this way to be snarky and a smart aleck, this can easily be confusing. People get it wrong all the time. Many incubators can't be programmed to count for you, certainly no broody hen. I believe you really need to be able to do this yourself.
To be honest, I can't either unless I do it my way.