The Start/End time setting is switched off by default. However, if you have enabled the setting on your account and decide to change it at a later point, it will not necessarily affect the previous time entries.
Figure: Change Start/End Time Setting by Navigating to Settings > Timesheet Settings
The Start/End time for previous timesheet entries will simply remain hidden unless you edit it. Any changes to the duration of an existing timesheet entry will remove the Start/End time details.