uidatepicker时间模式如何添加元素？

I have an uidatepicker setted in time mode, so it shows only hours from 00:00 to 23:59 . I wanna insert an element before the 00:00 something like "-- : --" , it must be the start values to detect when a user don't want to use the picker. There's a way to achive this behaviour?I have an uidatepicker setted in time mode, so