There is a defined limit to the number of days in the past you can confirm completion of tasks, as default this is 14 days. You can change the number of days as required, and you could if needed change, add the completed task and change back. The setting can be found at Settings > Settings > General Settings see picture below
Note: This setting helps ensure that completed tasks are mark as completed at the time of completion, as tasks can be marked in the Web or Mobile applications it should be easy for the person performing the maintenance task to do immediately, and you have a clear overview of activities.
