In order to work properly, RadDatePicker must not have its textbox and popup button hidden with display:none.
Here is a code library example, demonstrating how to make RadDateTimePicker work with all its elements invisible:
How to Use RadDateTimePicker Without a Textbox And Popup Buttons
If your scenario is different and you can't use the provided example, explain us what exactly are you trying to do and we will give further advice.
the Telerik team
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