It's more clear now: Clock, you mean Time, not SYSCLK.
We provided a detailed RTC examples (AN4759) which can give you better idea, or you can get inspired from RTC examples within STM32CubeF7 MCU firmware and use the smooth calibration function (HAL_RTCEx_SetSmoothCalib) to calibrate the RTC: STM32Cube_FW_F7_V1.16.0\Projects\STM32F746ZG-Nucleo\Examples\RTC\RTC_Calendar
Have a look at the "RTC smooth digital calibration" section in the RM0385 and this stm32f7xx_hal_rtc_ex.c file, this will help you to understand RTC calibration behaviour and have more idea on how configuration/implementation is done.
Hope this helps you.
Imen
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