It’s now 6 hours after I changed the calibration and the clock is still on spot. With the previous setting, it would be already 3 seconds off. I’m not sure how to do that in the Arduino environment. But it seems as if the import the Atmel header files, and then it’s simply:
int8_t value;
RTC->MODE2.FREQCORR.reg = value;
With value being a number between -128 and +127. Positive values make the clock slower, negative ones faster, and the value is about the ppm change to expect. 16 s/day is something like 180ppm off. So you cannot compensate that completely.