Updated for Synergist 12.5
Typically, it is considered best practice to submit/post time accumulated on your weekly time sheet at the end of each day. Once this is posted, the cells grey out.
However, if you work on a current job AFTER submitting/posting all the timesheets for the day, it is possible to add an extra timesheet from within the Weekly time sheet interface.
Weekly time sheet
Once submitted / posted, you can hover over the time values and the system displays ‘click to add another timesheet':
Adding an additional time sheet
You can then either add in additional units or negative units to subtract. Then, submit / post with comments: