A deadline can be set for a task. This deadline MUST be consistant with task start date against task estimated duration :

• If start date is defined, deadline cannot occur before the start date summed with expected duration.
• If start date is not enabled, it will nevertheless be calculated automatically as deadline minus the expected duration (0 as default for duration).
• If the task is assigned to a dated milestone, deadline cannot overcome milestone deadline.
• Deadline cannot overcome project end date.