luxy:
Btw, is ok to use time functions, based on both, millis() and RTC or is better to use only RTC for all time operations in that case (when RTC is already integrated)? ..or it doesn't really matter?
It depends on your requirements. In my mind the main difference between an RTC and millis (actually one of the build-in timers of the microcontroller) is that the RTC keeps track of date & time even if it is not powered up (assuming of course that it is connected to a back-up battery). The internal timers (hence millis) get initialized to 0 after a reset/powerup. So I would use an RTC if I want to date/time stamp events in a log, write an alarm clock or something similar that should not loose the date/time setting even if external power is interrupted. If on the other hand I want to blink an LED at a specific interval but don't care exactly what the time is when the blink starts I would go with millis or an interrupt based on a build-in timer in the Arduino.