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I have a little cardboard sign (folded triangle) on top of the incubator with a marker written: started_____, hatch due_____,off turner______, I use a piece of duct tape over the space and write the date, then remove or tape over the space for the dates for the next hatch. 21 days for chickens, 28 for guineas. I often start the guinea eggs, then 7 days later add a few chicken eggs so to have them all hatch together and raise. This helps bond/imprint the guineas to keep them returning to the run after roaming for bugs during the day.
You shouldn't worry so much about what day you're on based on time. About three days before the hatch is due, stop turning the eggs and "lockdown" until the hatch is finished, typically 24 hours after the first pipping is out.