Here is what we write in the PMP Exam Formula Study Guide about this. Please see

www.pm-formulas.com
for the guide:

There are two approaches for calculating ES, EF, LS and LF:

• First approach: You calculate the network diagram starting on day 0

• Second approach: You calculate the network diagram starting on day 1

In the PMP Exam Formula Study Guide we use the second approach, because when your sponsor tells you that your project starts on the first day of September, then that is September 1, not September 0. This is also the way that all modern scheduling tools seem to work. You schedule your project based on a calendar start date and not "on day 0".

That is why there is a slight difference between the calculations: you have to add/subtract 1 from the results in the second approach.

Of course, this often leads to confusion for PMP exam students and they ask which formula should we use on the exam?

We have discussed this with a number of PMP trainer colleagues and they agree that PMI does not "support" a specific method of calculating a network diagram. (Remember that next to the two options shown above you could also calculate a network path starting on a specific calendar date in hours instead of days, making the calculations even more complex).

Neelesh Pandey, PMP (a PMP trainer) has told us the following about his teaching experience with these formulas:

I use a PowerPoint presentation with animations to prove that no matter what method you follow, the result is same. I choose a part of a network diagram with four sequential activities, which sum up to a duration of 10. This path has a float of two based which we calculate LS and LF. My participants once assured that it doesn't make a difference tend to use the "zero" method. Somehow they find it simple as no subtraction is needed. PPT animation helps me a lot and also I ask my participants to calculate ES, LS, EF, and LF for a simple network diagram using both the methods.

As you see, both of these calculations will lead to the correct answer. However, in the exam the big difference is that the first approach (starting on day 0) involves fewer calculations because you don't have to "+1 or -1" each time. So, in order to reduce your "risk" of doing a calculation wrong and saving time during the exam, you might want to initiate the network diagram with day 0. However, in "real life" starting with day 1 is more appropriate.

Since PMI is aware of these varying methods, you should not see a question on the exam where only the application of one or the other leads to the correct answer.