Available with the 2016 R2 Release.

Available in LIB version 2016.3.1017, it will be also available in the 2016 R3 SP1 release.

When DateSelectionMode is Month, RadDateTimePicker has been opened and theme is changed, there is an InvalidOperationException.

Workaround:
1. Set the DateSelection mode to Day - <pickerInstance>.DateSelectionMode = DateSelectionMode.Day;
2. Clearthe merged dictionaries for the theme
3. Add the new dictionaries
4. Set the DateSelection mode to desired -  <pickerInstance>.DateSelectionMode = DateSelectionMode.Month;



Available in LIB version: 2017.2.710

The current workaround is to make sure that both the day and month include leading zeros in the ShortDatePattern:

            this.DatePicker.Culture.DateTimeFormat.ShortDatePattern = "yyyy.MM.dd";

The CLR property is named IsDropDownOpen, but the DependencyProperty has a type and it is named IsDownOpen. This works at runtime, but it brings an error at design-time.

The error says: "The property "IsDropDownOpen" is not a DependencyProperty. (some additional description here)"

If you set the TimeInterval of the RadDateTimePicker to a short interval, this will generate a lot of items in the clock area (the right section of the drop down, next to the calendar). If the items have not enough space to draw properly, they will start overlapping (thus clipping) one another. 
Add a way to enable a ScrollViewer for these situations.

Currently, this can be done by re-templating the RadClock control, as shown in the following KB article.

Add a property that formats the time items of the clock displayed in the DateTimePicker. Currently, the format is hard-coded to "t" which displays only the time part of the DateTime object. Adding this will allow to show also seconds and other parts of the date-time when it makes sense.