Each time you start the program, today's date is the session opened (as displayed at the small bar at the bottom-left corner of the screen). To change this date, you can click on the arrows on either side of the date, or click on the date itself to open up the Calendar.
With the calendar, you can select any one day to be opened. To change the month, use the arrows on either side of the "Month" display (top-left), to change the year, use the arrows on either side of the "Year" display (top-right). To change a day, just click on it, the calendar will close and bring you back to the kitchen with the selected date displayed.
