The accuracy of RTC component is defined by the accuracy of the connected external clock source. Can you please let us know the clock source you are using to provide input to the RTC component. The RTC component is driven by the Clk_Bak clock source. It is recommended to drive the Clk_Bak by the WCO, if it present in design, instead of the ILO. In case WCO is absent on the board you can use an external crystal (32.768-kHz) with high accuracy.
The RTC inaccuracy is ±1minutes/year (room temperature) if RTC clock source is ±2ppm calibrated.