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 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 the system displays ‘Click to add another timesheet:
Adding an additional time sheet
You can then add in additional units and comments to submit / post: