 # PMP Exam Tip: Calculating ES and FS We recently received the following question from a PMP Exam Student:

“There's some confusion in my head regarding some of the network diagram calculations. I'm lead to believe there are actually two methods to calculate Early Start (ES), Early Finish (EF), Late Start (LS), and  Late Finish (LF).

The first method adds or subtracts 1 where applicable. This assumes that the start activity has ES, EF, LS and LF as 1. The second method assumes the start activity has zeros for all values thereby not having to add or subtract 1 to any of the formulas.

Is there any indication in the exam that would lead me into knowing which "method" is being utilized so that I can apply the right formula?”

I answered that he is correct. there are indeed 2 approaches:
- First approach: You calculate the network diagram starting on Day 0
- Second approach: You calculate the network diagram starting on Day 1

I personally use the second approach, because when my sponsor tells me, that my project starts on the first day of September, then that is September 1 and 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 2nd approach). However, don't worry about this for the exam too much. The way that the question is formulated you should be able to identify how to go about this. Also: I understand that in most cases when you have to calculate this, it is the end result that is important and not how you got there.