How are you powering down? The RTC should maintain across a ''STANDBY'' operation, provided you identify PWR_FLAG_SB state in your RTC configuration code, and don't stomp on the RTC by reinitializing it when the STM32 comes back up from this state.
Use PWR_EnterSTANDBYMode(); to get the STM32 into it's minimal power state.
Tips, Buy me a coffee, or three.. PayPal Venmo (See Profile) Up vote any posts that you find helpful, it shows what's working..