As you work on the bug you can modify the start and finish time of your current work period. This can be necessary if, for example, you forgot to start time tracking when you started working on the bug, or vice versa - forgot to stop it when you actually stopped working on the bug.
To adjust the work start time:
Once the work is stopped and the record is saved to the timesheet, it can be edited in the Edit and Publish Time window. See the Timesheets Publishing section for the details.