Click on the date link (by default this will show today's date). A miniature, popup calendar will be displayed from which you may choose the date for the calendar item to be added.
You may move forwards and backwards by year and month using the indicated navigation buttons. When you click on the required date in the mini-calendar the date is set accordingly.
Complete the remainder of the form as indicated. The title is mandatory, but the calendar item content is optional.
Press Create to add the item to the calendar. The Update Preview button may be used to see what the entry will look like before committing it.