In order to enable the 12 hour clock format you need to set the DateFormat (and DisplayDateFormat optionally) properties of the embedded DateInput control. In addition if you want to switch the popup view too, you need to set the TimeFormat property of the embedded TimeView control. Please note that AM/PM designators appearance depends on the CultureInfo object set to the Culture property of the picker.
Please take a look to the following link for additional information: