Sept 1 to Sept 2 = day 1 at 10PM so tonight at 10 pm is day 4 so day 18 is the 19th at 10 pm and day 21 is the 22nd at 10 pm. You may have some hatching on the 21st to the 23rd or 24th depending on your temperature and humidity. My humidity was high for some ducks I incubated and they hatched out 2 days early (ducks take 28 days) and hatched to a day late. My chicks I just hatched began hatching on the 20th day and hatched until the 22nd day and my humidity was ok. I had some chicks under some broodies and theirs did the same. Some are faster developers and some are slower. Hope that helps.