Lock time editing
Admins have a right to lock time editing for regular members after a certain period of time or by approving a timesheet. In this case, the time will be grayed with the reason showing via a tooltip.
Lock editing after a number of days
For example, if the limit is set to 2 days and today is Friday, you can only edit the time for Friday and Thursday. Wednesday data is no longer editable.
As an admin, go to Team Settings page and select 'Don't allow time tracking after X days' in the 'Tracking Policy' block. The option can be applied to members, but not admins or supervisors.
Lock editing of approved time
If your team enables 'Time Approval', any time editing will also be limited for approved timesheet. It also becomes grayed out and there is a reason showing via a tooltip.