I need to skip the inputBox focus on Tab in order to speed up data entry through keyboard, but even if we set IsTabStop property to false on the DatePicker, the InputBox on its left is still getting focus on Tab, is it possible to change this behaviour ?
To skip the focus on the DatePicker, you have to set the IsTabStop property to false to the PickerTextBox (PickerTextBox is part of the DatePicker).To get the PickerTextBox template you can create one DatePicker in Blend, and then right-click on it and Edit Template. After that Blend will generate all templates for you and after finding the one of the PickerTextBox you can set the property there. You can see the attached project for more details.
If you have any need of further assistance please feel free to ask again.