I've figured most of it out, literally all I need now is help calculating the days passed since 1/21/2015. I know the theory, but I don't know what code to use. Any ideas?
Edit:
I realized that what I should do is calculate the unixtime from 1970 to present date, than the unixtime from the first date than subtract the two and the difference (if i divide each one by 86400) will give me the amount of days from the first date to present. Now all I need to do is program it.
Edit 2:
I can't believe I didn't know anything about programming this morning... I added a variable in setup:
firstDate = DateTime(2015, 1, 21, 0, 0, 0);
and I did the following operation in the loop in order to calculate days since than:
DateTime now = rtc.now()
Serial.print("Days: ")
Serial.println((now.unixtime() / 86400L) - (firstDate.unixtime() / 86400L));
Now what?