I agree with Steve in the sense that the PMP Exam does not have a preference. And based on many discussions I've had with other PMP trainers over the years I have come to the conclusion that you will arrive at the correct answer, no matter if you use "0" or "1" during the exam.
However, I disagree with Steve on personal preference. I prefer to start with "1" because that makes more sense to me.
If you are a customer of our
PMP Exam Formula Study Guide
, then please take a look at the section titled "There are two approaches for calculating ES, EF, LS and LF" where we explain this preference in detail. (In a nutshell: First all modern scheduling tools start on day 1, and second when my boss tells me that my project starts on the first day of August, then that is August 1 and not August 0.)
So as you can see... using 0/1 is a matter of almost "religious preference" and the good news is that in this particular case the PMP Exam is agnostic.