Thank you,I had the same problem with counting days until an old timer explained it to me. This is what they said:
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.) You set eggs at 8:00pm on Saturday, March 9th so your hatch day (Day 21) is Saturday, March 30th. Another way to look at it is in hours. Saturday, 3/9 at 8pm is 0 hours. 3/10 at 8am is 12 hours and 8pm 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 evening 3/27 @ 7:59pm. Day 19 starts @ 8:00pm Wednesday and lockdown begins. Day 20 starts @ 8pm on Thursday and Day 21 starts @ 8pm on Friday and ends on Saturday @ 7:59pm. So on Saturday at 7:59pm you have had 21 days (each consisting of 24 hours) of full development.
I had started a thread asking the same question some time ago. I have just quoted your post and stuck it in the same thread, so if anyone ever find my question, they will get your answer!
Thanks again,
Ben