Ensure that the resources you plan to assign to a task are completely available on the start date to avoid unnecessary delays. Project plans and schedules get delayed one day at a time. If there is a delay right in the beginning, it might have a cascading effect on the entire schedule.
Only an Admin, Secondary Admin and Project Manager can change the start date of a project anytime during the execution.
By default, the start date is the date on which a project/task group/task is created.

Due Date

The Due Date is the last or final date associated with a project or a task. All work should be completed on or before this date. Duration of a task is the number of days between the due date and the start date.

Try to avoid specifying unrealistic due dates.
Constantly review the due date at every phase of project execution. If road blocks occur you can easily change the due dates or resources.
By default, the due date for a task is 2 days after the date on which a task group/task is created. For a project, the due date defaults to 90 days after the creation date.
When a task is not completed before the due date it becomes overdue.
When a project has exceeded its due date and the task is yet to be completed then extra time gets allocated to complete the project. The time period from the project's due date to the extra time allocated to complete it, is considered as the "Critical Time Period". A red line appears in the Gantt chart, after the project bar indicating the Critical Time Period.