The default Kentico Calendar does not have the ability to set the returned input. Here are your options:
- Set the form control to Textbox instead of Calendar, then on the form itself utilize your own Date picker jQuery module, there are many out there that you can control the displayed value (like Bootstrap DatePicker), just be sure that the displayed value can be DateTime.Parse() properly into a date time if the field is going to be of type Date/DateTime
- Create your own form control that acts as you wish, sadly the existing Calendar form control itself does not have any options within the control to alter the rendering.
I suggest option #1!