年月日选择器结束时间问题

开发者您好，目前发现如果setSelectedItem和setDateRangeEnd这两个如果设置为同一年，会导致结束时间的月日不起作用，不管设置为多少都显示12月31，应该是bug

Start by locating the DatePicker entry points setSelectedItem and setDateRangeEnd. Reproduce a range whose start and end years match, then trace why the configured end month and day become December 31. Done means same-year ranges preserve the requested end month and day, with a regression test if the existing picker tests cover this behavior.

Written by the indexing model from the issue text.