The board which is made with Atmega operates on battery, it should turn on every 6 hours and log some data, then go back to deep sleep to save battery consumption. Atmega32 is used and not Atiny because of complexity of tasks and sensors connected to the circuit.
What I have found so far, is that I can keep track of time using an RTC IC.
I've read datasheets of some RTC IC's but they don't provide any such capability as generating a signal for Atmega's external interrupt at specific intervals, so it can wake up from deep sleep. (Something similar to cron-task in *nix systems is needed, if you are familiar with it.)
And of course search terms like "Atmega scheduled task" and alike did not bring up anything.
How can I efficiently (in terms of power consumption) wake Atmega up from deep sleep periodically?