Expected behavior

Until a full date* (year, month and day) was selected, there shouldn't be any date displayed in the header of the dialog (it should be blank) and the OK button should be disabled.

Picking a year or month should not behave differently than paging through the calendar using the < and > buttons and should not alter the date displayed in the header.

Actual behavior

On the example page in the Basic example case with the blank input, open the Datepicker. Its header will show today's date even though no date was selected yet. Without clicking on any date, press OK. The dialog closes but the input is still blank. This is misleading because given the fact that the header of the dialog showed today's date, one would expect that today's date is going to be inserted.

Open the dialog again and use the < and > arrows to page to other months. Notice that the date in the header does not change and remains today's date. Clicking OK will again not insert any date into the input.

Open the dialog one more time and now click on the month and year button to open the year view, then pick a year. This action changes the date in the header to have the selected year, but this does not make sense - selecting a year or a month should not behave differently than paging through the calendar using < and >. If you press OK now, a date will be inserted.

*) I opened another ticket where I asked that the Datepicker should also support selecting years or months. In case that will be implemented, clicking on a year or month respectively would also count as having selected a date because it would be the last step in your action of picking a date.