Date fields with a popup widget also allows free text input of values in the actual textfield. If you have your formatter set to accept dates in the format dd/mm/yyyy, but enter a date like dd/mm/yy in the text field, the popup calendar automatically selects the correct year, but tabbing out of the field does not complete the years missing characters.

eg.

1) configure date field to accept format dd/mm/yyyy, with popp widget
2) in the date textfield enter 12/12/81 (note, 81, instead of 1981)
3) notice that the calendar popup selects the correct day, month and year
4) tab out of the field
5) notice that the date is still set to 12/12/81 - which is invalid (should be 12/12/1981)