# Determine the True Length of a Schedule with Slipped Tasks and Inflexible Constraints

As you track progress by entering actual start and finish dates for tasks, you may find that the project finish date remains the same, even when a number of tasks have been completed later or have taken longer than originally planned. The reason could be that one or more tasks in your schedule have an inflexible constraint (Must Start On, Must Finish On, Start No Later Than, Finish No Later Than) applied.

A task with an inflexible constraint doesn’t move, even if predecessor tasks finish later than planned. A task with a Must Start On constraint, for example, doesn’t get "pushed" to a later start date if tasks that precede it finish later than their original finish dates. Instead, it remains firmly anchored to its specified start date while predecessor tasks move.

The tasks that follow the inflexible task remain unaffected by the late-starting tasks. Therefore, even though tasks that precede the inflexible task are late, and the project as a whole is late, Microsoft Project may show your schedule incorrectly on the original project finish date. Because of the task with the Must Start On constraint, Microsoft Project can’t recalculate a more realistic project finish date.

What do you do if you want to know the "real" project finish date? You can allow the inflexible task to move without removing its Must Start On constraint. You can choose to make schedule changes override constraints.

By default, Microsoft Project honors constraint dates. The start and finish dates of a linked task with an inflexible constraint, such as Must Start On, stay the same even if the schedule changes. If you want Microsoft Project to automatically recalculate a constrained task’s start and finish dates when the schedule changes, you can choose to have schedule changes override constraint dates. Because tasks with flexible constraints such as As Soon As Possible and As Late As Possible have no fixed dates to honor, they behave the same way regardless of the option you choose.

When you use constraints in your project plan, you can choose to have schedule changes override constraints or have constraints override schedule changes — for all affected tasks. A project plan cannot accommodate both options at the same time.

To set constraints so schedule changes can override them

1. On the Tools menu, click Options.
2. Click the Schedule tab.
3. Clear the Tasks will always honor their constraint dates check box.

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