The problem should be related with the registration of the DatePicker you use or the implementation. Make sure
you use the the sample from the latest SDK(1501). Add existing project to your Visual Studio solution and select the sample from the SDK. Build the DatePicker sample and add a reference to SitefinityWebApp project. Build the SitefinityWebApp and view it in the browser. Register the control as described in the documentation
With this registration you will have the control in pages. To add it in Forms module register it the same way you register it in pages but add it in the toolbox Form Controls. This should get you the control as it should be.
the Telerik team
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system and vote to affect the priority of the items