Am I countin my days incorrectly??

[Yard full o' rocks]

I am wondering if I am counting my days incorrectly to the hatching date. Should chicks be due "during" the 21st day of incubation, or AFTER 21 complete days of incubation? For example, if I started eggs at 12midnight on 5/5 (Thurs nite.Fri morn), I would expect them any time after 12 midnight (Thurs nite. Fri morn) 5/26. Is my calculation right OR should they be due after 21 COMPLETE days of incubation therefore I should not expect them until after midnight 5/27 (Fri nite/Sat morn)??

 

[Chicky Tocks]

For example, if I started eggs at 12midnight on 5/5 (Thurs nite.Fri morn), I would expect them any time after 12 midnight (Thurs nite. Fri morn) 5/26.

This is correct. However, none of my hatches have held out until the 21st day. My chicks tend to come early on the 19th or 20th day. I've heard others say theirs are late.​

 

[Chicks_N_Horses]

Well mine always hatch 2 days early...every single time. My temps. must run a little high. I even ran it a degree less this time around and still they hatched 2 days early.....