Tasks can sometimes start and complete late without impacting the Completion Date of the project.
The Slack Date displays the exact date when a task could definitely impact the Completion Date of the project.
The Slack Date is different than the Projected Completion Date, as the predecessor relationships and task constraints increase the slack time.
For tasks that are on the Critical Path of the project, the Slack Date matches the Projected Completion Date of the task unless the Progress Status of the task is already Late or Behind.
For more information about the Critical Path, see "Understanding the Critical Path."
For tasks that are not on the Critical Path, the slack time increases the earlier the task is scheduled. For these tasks, the Slack Date remains in the future until the tasks become so late that they begin to impact the Completion Date of the project.