Must start/finish on constraints with a twist

Hi readers,

A while back I got an interesting reply on one of my blog posts. A top contributor of one of the LinkedIn forums I frequently visit had an interesting view on the Must Start/finish constraints that I was not aware of yet. So here is a post dedicated to Sai Prasad describing this behavior.

Hard constraints

The constraints “Must start” or “Must finish” are shown up in a schedule on the indicator column as a red calendar.

On normal occasions these constraints are really hard constraints. Ergo, tasks will not move based on the dynamic schedule and will ignore dependency’s even though there are dependency’s that would normally change the start or finish dates of the task.

Not so hard constraints

Sai started his comment on my post with this: “I want to add a couple of points to so that you can recreate the same behavior of deadline in must-finish-on constraint but with minor variations.”. And I did recreate the behavior and I was amazed! “Not so hard” constraints,,, who would have thought.

Here is the rest of the information Sai provided, have a go at it yourself:

Whether a task has must-finish-on constraint or deadline set, inserting a Total Slack column in the GANTT-chart view can tell by how many days the affected task and its predecessors should start ahead.

For a task with deadline set, its late finish date is set to the deadline and all its predecessor’s late start and late finish is calculate based on this date. If this task finishes after the deadline date, the total slack of this task will be negative for the current task and its predecessors. Because the late finish is deadline date and early finish is past the deadline date.

Now, let’s see must-finish-on constraint. For a task with must-finish-on constraint set, its early finish date and late finish is set to the constraint date (and not its predecessors). If this task finishes after the must-finish-on constraint, Project has a scheduling conflict because the total slack is now negative. To avoid this scheduling conflict, you can configure Project to schedule task according to the links with the other task through File backstage, Project Options, Schedule and clear “Tasks will always honor their constraint dates” in Scheduling options tab:

After clearing this option, project displays a “not so hard” constraint: