That is how I do my math. There is no day 0 on the calendar.Whatever day of the week you set your eggs on is the same day you can expect them to start hatching. (Sometimes earlier. Sometimes later.) We set eggs at 12:00pm on Saturday, March 9th so our hatch day (Day 21) is Saturday, March 30th. Another way to look at it is in hours. Saturday, 3/9 at 12pm is 0 hours. 3/10 at 12am is 12 hours and 12pm on 3/10 is 24 hours. 24 hours = 1 day or Day 1 of incubation. When you read or hear "lock down or stop turning" on Day 18 it means after the full 24 hours of Day 18 has gone by you stop turning. So that would mean that Day 18 ends on Wednesday morning 3/27 @ 11:59am. Day 19 starts @ 12:00pm Wednesday and lockdown begins. Day 20 starts @ 12pm on Thursday and Day 21 starts @ 12pm on Friday and ends on Saturday @ 11:59am. So on Saturday at 11:59am you have had 21 days (each consisting of 24 hours) of full development.