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The RadDatePicker control is intended to select only one date from the Calendar. If you would like to have date range there are couple of options.
The first option is to use two RadDatePicker controls - one for start date and one for end date. Similar approach is illustrated in this online demo
Another approach would be to use RadCalendar control placed in a popup container. This way the RadCalendar will not be always visible on the page.
I hope this information would be helpful to you.
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