TOPIC: Minus /Plus in Duration equation

Hi ,

I went through webcast and understand that +1 and -1 point of view.

My question let look at this link:

en.wikipedia.org/wiki/Program_Evaluation_and_Review_Technique

Why such equations that is :

EF = ES + Duration +1

LS = LF -Duration -1

LF = LS of successor -1
ES = EF of predessor +1

From precast is clear how come it is not clear when I see different diagram.

My only explaination is starting first day is day 0 not day 1 which makes is so different from prepcast.

Thank you.

saad