Lets look at 2 similar types of constraints, but when I say similar, I mean similar in the way that it affects the activities and floats,

1. Project Must Finish By
2. Mandatory Start / Mandatory Finish

1. Project Must Finish By:

This constraint is applied at project level in the project details tab. When we apply this constraint it fixes the Late Date of the last activity of the project and hence it affects the calculations during the backward pass. Let’s take a look at the following example;

In the above scenario the project finishes on 6th day, but we would want the project to finish on the 5th day. Hence we apply Project Must Finish By = 5 and schedule it. During backward pass the constraint kicks in and forces the last activity C to finish it on 5th day and calculates from there on. Hence we get a total float of -1 on the activities.

Now what do we do with a Total Float = -1?

While monitoring projects with fixed Finish Deadline, it helps us to understand that all the activities with a Total Float of -1 will need to finish 1 day before the Early Finish (EF) date and hence our resources need to be planned accordingly. It also helps us in similar way if we are creating ‘What-if’ scenarios, cause we would know what activities do we need to expedite and whether we would be able to achieve the productivity required to match the reduced durations.

2. Mandatory Start / Mandatory Finish

This constraint can be used in situations where one particular activity in a project has a fixed Start/Finish date. Once we impose this constraint on the activity it affects both Early as well as Late dates. Let’s take a look at the example below:

In this example Activity B has a Mandatory Start Constraint on day 2. What this does is that the LS & ES dates of activity B get fixed at day 2. Hence during Forward Pass the Early Start of Activity B gets fixed at 2 and due to this the activities which are dependent on B get affected. During backward pass the same process is followed and the activities preceding B are also affected where Total Float of Activity A becomes -1 even though the constraint is on Activity B. While it is okay to use Mandatory Start/Finish constraint, we need to understand its effects on Preceding and Succeeding activities before using it.