Within Teamwork, you can edit the date that a milestone was marked as completed.
Note: Only site administrators, project administrators, or people with permission to edit milestones can change the completed date.
To change a milestone's completion date, go to the Milestones area of a project and select the Completed tab. Click the name of the milestone you want to update to go to the milestone details view.
In the milestone details pane on the left-hand side, hover over Date Completed and click Modify.
In the modify modal, you can change the person who completed the milestone, as well as the date the milestone was completed.
Click Apply Changes to save the new milestone properties.
You can also access a milestone's details view via the Milestones section of the Everything area.
Clicking a milestone here will open it in its associated project where you can modify the completed date.
For more information, see: Editing a Milestone Date