TAB key is a special key in WinForms - it is used for navigation between controls.
To avoid the default WinForms behavior you, should override Form's ProcessCmdKey event and check if Keys.Tab is pressed. Then, you should select next date/time part in the RadMaskedEditBox/RadDateTimePicker. Please refer to the sample code below:
MaskDateTimeProvider provider =this.radMaskedEditBox1.MaskedEditBoxElement.Provider
Take a look at the attached sample project that demonstrates the complete behavior for RadMaskedEditBox and for RadDateTimePicker navigation.
I hope this helps.
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.