The Actual Completion Date is located within the Project Details, Task Details, Task Update, Issue Details, and Issue Update Pages.
The Actual Completion Date represents the date and time the work is completed. When an item is marked as Done and is set to the status of Complete it will set the Actual Completion Date as the date of the change in status. If that date is not an accurate reflection of when the item was actually completed then you can amend the Actual Completion Date by Editing the item.
The System Administrator can identify whether today's date or the Planned Start/Completion Date will be used when the Task is set to New, In Progress, or Complete using Preferences.
The value for the Actual Completion Date is always the current date and time. A user who updates the task or issue after the work is completed should backdate the actual completion date value for a more accurate Actual Duration calculation. Inaccurate dates will adversely affect the Actual Duration of the task, issue, or project.
NOTE: The latest Actual Completion Date from a project's tasks will be used as the Actual Completion Date for the entire project.