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Q&A on 2 methods in Calculating ES and LS

This is an interesting conversation on critical path calculation methods – in particular ES and LS. Mahendran asked the questions and Mike Newell (noted PMP book author) and Dr PD Giammolvo provided the answers


 The question is regarding the method to be adhered to calculate Early Start(ES) and Late Start(LS).

  a) In some books(Rita Mulcay) the ES of
the first activity is taken as 0(zero) and early finish is
calculated as ES + duration = Early Finish

  b) In some books(Head First) the ES of
the first activity is taken as 1(one) and the early finish
is calculated as ES + duration – 1 = Early Finish.

When the question is regarding “What is the ES of an
activity”. Both the books give different answers.

The answer in the first book is X and the second book is
X+1.  Could any body please clarify which is correct.

Mike Newell answer:

There is a convention that all schedulers use if they are working in days and most schedules are in days.  Some are not. in days then.  All activities start on the morning of the day they start and end on the afternoon of the day they finish.  Thus, if an activity took one day, it would start on, say, March 14, in the morning, and it would finish on March 14, in the afternoon, and from morning of the day to afternoon of the same day is one day duration.

If you are scheduling in hours for example, the clock method is used.  This says that if an activity takes one unit of time, in this case an hour, you think of the hands of a clock.  In one unit of time you go from 12 o’clock to one o’clock and the next unit starts at one o’clock.

So, you see both methods are used.  If time units are days, you use one method and if you are using any other time unit, you use the clock method.

Incidentally, Microsoft Project does it this way.  Try it.

Try not to memorize formulas, reason things out and you will remember them long after the exam.

If you want, contact me at [email protected]


As a follow on elaboration to Mike’s answer, starting the clock running at
zero is used by ADM or Activity on Arrow (AoA), while the clock starting to
run on the morning of day 1 is the convention used by PDM or Activity on
Node (AoN).

You will find that Primavera, MSP and nearly all other software today uses
PDM. (I am unaware of ANY major software that uses the ADM or AoA method)  I
BELIEVE, (but am not 100% certain) that PMI mercifully killed off ADM/AoA
and I would hope that AACE and other professional organizations do likewise.

Not to date myself, but for back ground information the last time I used ADM
professionally on a project was in 1979-1982 on the Negev Airbase Project,
Ovda, Israel. We were running a program called MSCS and to give you some
idea just how long ago that was, we were updating the schedule monthly using

When Primavera went from being DOS based to Windows, they killed off the
option of using ADM or PDM, and use PDM only now for many years.

To my knowledge, ADM/AoA only exists in textbooks these days..


Dr. PDG, in Boston


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