Incubation question

I have a note book and mark 1-21, I write the temp, humidity and time in every day to keep track of the eggs through incubation.

I fill in day 1 when the eggs have been in 24 hours, eggs can hatch anywhere from day 19 to day 23, some times event sooner or later. It us said this is due to the incubator being a bit hotter if hatch is early or cooler if hatch is late. Personally I have found it pretty much depends on the batch of eggs.

I even had one chick hatch day 18, the incubator was running sweet and the other 20 started to hatch day 21 

Thanks guys, i've hatched several batches of eggs in the past but for the life of me tonight i couldn't figure out whether tomorrow is day 18 or 19