An ArgumentOutOfRangeException is thrown when the Separator property is set to a string that contains alpha-numeric/numeric "not required" mask tokens, and the clear button is pressed. The control works with a custom RadMaskedTextInput control to parse different date and time patterns for the start and end dates, which replaces the mask tokens with a placeholder, resulting in the exception when updating the Value property of the RadMaskedTextInput when the value is cleared.

Currently, you can select a single day, but this will set the StartDate to the selected date and EndDate to null. Add option to enable a selection range of a single day, where StartDate == EndDate. 
At the moment, you can do that by programmatically setting the StartEnd and EndDate properties, or by editing the values in the text input, but this cannot be done via the UI. 
Also, when StartDate == EndDate, a click on another date in the calendar continues the selection, instead of starting a new selection range. In this situation, a new selection should be started when another date is clicked.

Currently, the Text property of the TextBlock elements in the top bar can be changed by extending the RadDateRangePicker control and by overriding the GetTemporaryStartDateRepresentation and GetTemporaryEndDateRepresentation methods. 

We could introduce properties for changing the date representation rather than extending and overriding the above-mentioned methods.