In my “minimalistic standalone ATmega328 powered by a capacitor” project I was walking from the start through first tests, reducing consumption, optimized tests and adding a RTC. The RTC has let me learn a lot about waking up from sleep via interrupt.
Now one more component is due to come: external EEPROM to extend data memory. As I’m hoping to extend total uptime more I probably will run out of EEPROM memory when keeping the basic example (reading one ADC channel once a second and writing data to EEPROM). Furthermore, when it comes to practical use of the system, internal EEPROM is too small also.