Concerning the RTC "calibration" - if the RTC does not work correctly, please check the clock settings of your MCU.
Concerning the 12 hour mode: In this case - the day still has 24 hours - you can adapt the method BezierClock::TimePicker.OnLoadHourItem() so that the strings of the list items are in the range from 0...12 instead of 0...23.
Btw: Within the Build Environment for STM32F429-Discovery, you will find also the exampled BezierClock - it is already adapted to work with the RTC.
Best regards,
Manfred.