Kenji Arai
In the past, you need modify rtc_api.c in mbed-dev source code. From this revision, you can just use RTC function all of conditions (Normal, Reset, Stand-by, Power OFF).
Note
From now on, you do NOT need any modification for mbed-dev library because STM team updates rtc_api.c source code and support RTC function under reset & standby condition includes power off condition (You need additional VBAT back-up hardware).
Please refer following NOTE information.
/users/kenjiArai/notebook/nucleo-series-rtc-control-under-power-onoff-and-re/