STMicroelectronics STM32F05 series Reference Manual page 554

Real-time clock (RTC)
Note: Bits 7, 6 and 4 of this register can be written in initialization mode only (RTC_ISR/INITF = 1).
It is recommended not to change the hour during the calendar hour increment as it could
mask the incrementation of the calendar hour.
ADD1H and SUB1H changes are effective in the next second.
This register is write protected. The write access procedure is described in
write protection on page
554/742
540.
Doc ID 018940 Rev 1
RM0091
RTC register