Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32l0/STM32L0x0/RTC/CALR#0x0
RTC calibration register
Calibration minus
Use a 16-second calibration cycle period
Use an 8-second calibration cycle period
Increase frequency of RTC by 488.5 ppm
https://github.com/modm-io/cmsis-svd-stm32