Summary:
One of the user in the organization has gone ahead and entered time on a future timesheet and saved the timesheet as well. After a few weeks, when this user wants to modify the time entered on the future timesheet, the values in the box are grayed out and he is unable to change them.

Resolution:
After the user had initially entered time on the future timesheet, the administrator had gone ahead and set an end date to this user. Since the end date falls before the future timesheet period, the timesheet cells are now grayed out. The user can no longer edit the time, however he can still submit the timesheet and also the approver can approve that timesheet.

Note:
One very important point to note here is that even the administrator will not be able to change the already entered time values by the user even if the administrator reopens this timesheet from Historical Timesheet page.