The backward pass starts at the end or due date of the project, and calculates a Late Start date and a Late Finish date for each task and milestone.

These are what we call the ‘Must happen’ dates. They are the latest dates that each task and milestone can start and finish without impacting the project completion. If any task or milestone misses its Late dates, the project will not finish on time. These dates  ‘Must happen’.

As described previously, in a standard schedule model, the Late dates are shown on the bottom of the task box.

A finish milestone is any milestone without a successor. You should only have one finish milestone (in our example – Target Complete).

A finish milestone must be given a finish date constraint to initiate the backward pass so that Microsoft Project knows when to schedule the Late Finish of the milestone. We will assign a finish date constraint to drive our backward pass.

Microsoft Project can determine the Late Finish of a finish milestone in one of three ways:

    1. A Must Finish On  Constraint Type may be set with the Constraint Date equal to the due date of the project. The backward pass will start from that date. This is the preferred method, and we will demonstrate why.

    2. A Deadline date may be assigned. This, however will only drive the backward pass if the Deadline is earlier than the Early Finish of the finish milestone. This is often the recommended method, but we will demonstrate why it is not the preferred method.

    3. If no finish date constraint is assigned, Microsoft Project will use the Early Finish of the finish milestone and assign it as the Late Finish of the finish milestone and start the backward pass with that date.

We will discuss and demonstrate the differences between these methods in the section on Constraints and Deadlines, as that is where the critical differences are. For now, we will continue on using the Must Finish On date approach.

The Must Finish On constraint is assigned to a milestone – called Target Complete in our example – that is the finish milestone in the project. The Target Complete is linked to a Project Complete milestone, with the Target Complete as the successor in a finish to start link.

Note: Must Finish On constraints should NEVER be assigned to a milestone that has a successor!

When you assign a Must Finish On constraint to a milestone, the Early Start, Early Finish, Late Start, and Late Finish dates are ALL set to the Must Finish On Constraint Date.

As described in the Schedule Model Paths section of Lesson 3, all projects should have a milestone at the end that everything flows into. The milestone that everything should flow into is the Project Complete milestone. What this means, is that all paths that have no other successors, should include a successor of Project Complete. The Project Complete milestone may have any name that best represents the completion of the work for your project.

The Project Complete milestone is linked finish to start to a Target Complete finish milestone. We recommend that you always use the name Target Complete for this milestone as that is what it represents and being consistent makes it easier to identify. If you choose to use a different Task Name, make sure to use the same name consistently across all projects.

Once your schedule model is set up this way, the Finish date (Early Finish) on the Project Complete milestone represents when the project is currently expected to be complete – the ‘Can happen’ date. The Finish date on the Target Complete milestone represents when the project is due –  the ‘Must happen’ date.

 
To set a Must Finish On date, access the Task Information dialog from the Task ribbon in the Properties section by clicking the Information button and then click the Advanced tab.

You may also access Task Information by double clicking on the milestone.

 
Task Information Dialog
The formulas for calculating Late Start and Late Finish are shown below. Because the backward pass calculates finish to start (right to left), the Late Start of a task is typically calculated from its Late Finish.

Then predecessor dates are calculated based on the successor.

  • Late Start (LS) = Late Finish (LF) – Duration + 1
  • LF of Predecessor (FS dependency) = LS of Successor – Lag – 1   (Lag should be 0 in a FS link)
  • LS of Predecessor (SS dependency) = LS of Successor  Lag
  • LF of Predecessor (FF dependency) = LF of Successor  Lag

If a task has multiple successors, the LF of the predecessor is driven by the earliest successor.

In the diagram below, if Task B has a Late Start at the beginning of 6/9, Task C has a Late Start at the beginning of 6/12, and Task D has a Late Start at the beginning of 6/4, in order to honor all of the links, the latest that Task A can finish (Late Finish) is at the end of 6/3 – before the earliest successor needs to start.

Let’s look at some calculation examples. To simplify our calculations, we will use day numbers instead of dates. We will also assume a seven day work week.

If a task has a Late Finish date of day 25, the formula to calculate its Late Start is LS = LF – Duration + 1 = 25 – 10 + 1 = 16. If the latest that the task can finish is the end of day 25, than the latest it can start is at the beginning of day 16.

For a finish to start link, the Late Finish of the predecessor (Task A) is equal to the Late Start of the successor – Lag (which should be 0 with the FS link) – 1.  16 – 0 + 1 = 15

We calculate the LS of Task A using the same formula we just used to calculate the LS above, LS = LF – DU + 1.  15 – 10 + 1 = 6

For a start to start link, the Late Start of the predecessor (Task A) is equal to the Late Start of the successor – Lag.  11 – 4 = 7

We calculate the LF of Task A using the formula of LF = LS + DU – 1.  7 + 10 – 1 = 16

For a finish to finish link, the Late Finish of the predecessor (Task A) is equal to the Late Finish of the successor – Lag.  17 – 5 + 7

We calculate the LS from the LF using the formula LS = LF – DU +1.  12 – 10 + 1 = 2

As described previously, if a task has multiple successors, its Late date (in this case Late Finish) is driven by the earliest successor.
Here is an example of a very small schedule model with two paths.

Notice that the backward pass is driven by the Must Finish On Constraint Type with a Constraint Date of 130. Milestone F has both a Late Finish and Late Start of 130 (the end of day 130 – from the MFO). Task E, the predecessor of milestone F, also has a Late Finish of 130, because F is a milestone.

Remember that milestones are the exception to the normal calculation formulas. They are 0 Duration, so no time passes and the predecessor Late Finish is equal to the successor start and finish.

Where there are multiple successors (Task A), all must be considered. The Late Finish of A is driven by the earliest successor (Task B).

Remember the phrase – ‘Earliest Late Date’.

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