Could the genius who designed the 85353 and the 163L if allowed to design any future AVR's please do an RTC using counters with perhaps an alarm interrupt, like most RTC designers seem to have done.
This new innovation would not need sleep/wake up, waste code and POWER simply counting seconds.
To say the 85353 and 163L have a RTC is misleading and deceptive.
It's akin to describing a fallen tree a home.
As things stand, it is simpler and cheaper to use an external RTC consuming less than 1ua without the need for any extra power up/down make sure it works circuitry.