Internally the PSoC 6 RTC only stores the year as a two digit BCD value (0-99); no century information is stored.

On RTC initialization the HAL must, as a result, assume a default century. If cyhal_rtc_write has been called with a different century than the default, its value must be stored and that value must persist through deep sleep, hibernate, software resets, etc. PSoC 6 hardware provides a number of BREG registers which exist in the BACKUP domain and will persist over these power modes and resets. The HAL uses the highest indexed BACKUP->BREG register to store the century for the RTC.

Therefore do not use the highest indexed BACKUP->BREG register as it is reserved for internal HAL usage.