MS Project: Sub-tasks causing unexpected 'Late Start'

Question

I'm attempting to build a schedule and use MS Project to calculate dates and flow, based on constraints and task linking, but am unable to overcome an apparent issue with how the 'Late Start' is calculated for sub-tasks.

Minimalist Example:

This is a simple schedule with two tasks, and a completion milestone, with a simple linear dependency structure, and a 'Start No Later Than' constraint on the final milestone. The 'Task Group' is a placeholder in this scenario.

The 'Late Start' dates calculated by Project here are working as desired and expected, moving back appropriately from the 'No Later Than' date.

Now, I move 'Task 2' to be a subtask:

Now, the 'Late Start' for tasks 1 and 2 are not what would be expected based on the task hierarchy and durations associated with the tasks.

Can someone help me to understand how I can create a set of grouped tasks that will set the 'Late Start' date appropriately, per the scenario above?

Thanks.

Answer 1

The general rule is that the late date of a task is the earliest of:

• Its successor late dates, adjusted for lag, calendars, and
  duration;
• Its Deadline, adjusted for calendars and duration;
• Its SNLT/FNLT/MSO/MFO Constraint dates, adjusted for calendars and
  duration;
• The task’s early dates, if it has no successors and “calculate multiple critical paths” is checked;
• The Project’s early finish date.

As an exception, top-level tasks (with no outline parent) ignore this last condition. While some advanced users might find the exception useful for tracking terminal float in a contract, I think it’s useless. In any case, the exception is confusing, leading to your misplaced expectations in the Minimalist example.

In MSP, there is no way to force a group of (subtask) milestones to recognize a late date that is later than the early finish date of the Project. You might consider the common method of adding a “target” milestone as an immediate successor to each of yours, then applying concurrent SNET constraints AND Deadlines to the new “target” milestones. That delays the Project’s early finish date enough to remove it from the late date calculations for your grouped tasks/milestones.