In order to generate shift in past, click on the left arrow or choose the past date by clicking "go to" button.
Once you are in past date, say 14th march (in example below). Click on "Add Shift" and generate shift. Please read program for further reading.
Alternatively, click on the past date in the calendar (15th Aug in example below), it will then prompt you to make a new program. You can create one off shift for that date, or you can generate program starting from that date.
Note: Do not click "Add Shift" button in order to generate past shift, as this will only allow you to select current and future dates.