It's not an option disconnect the actual completion date with the task being marked complete as they are directly related to one another.
- If the project is still open users should still be able to log hours on completed tasks from the tasks themselves. However, if the project is closed there is a setting in the backend that you can turn on allowing users to log time on completed projects.
- If they are logging time from the timesheets area make sure that the setting in timesheet preferences allows for complete work to populate.
https://experienceleague.adobe.com/en/docs/workfront/using/administration-and-setup/set-up-wf/config...
5. In the Where users can log time section, configure any of the following options:
6. In the Pre-populate timesheets section, configure any of the following options: