I was interested in putting the core in sleep mode (to save power) while the RTC or a Timer is still running and wake up when the RTC/Timer expires.
I read this is only possible with Arduino Zero, but I found other micros such as ATMEGA328 and ATMEGA1284 also have RTC and the Datasheet shows low power mode numbers while RTC is still running, so I'm wondering if this could also be achieved with classic Arduino boards having those micros...
Simply put YES.
There are a myriad of libs and functions available for a variety of boards.
Simply type the name of your board and the word sleep or watchdog into google eg "arduino uno sleep"