So... the calculator is telling me that using 6 WD 8TB Reds, in RAID-Z2, that I have a TTDL of 2.4 x 10^11 h that's... 27 million years. lol Is that calculation correct?
Yes, but actually it will be lower because I didn't take into account everything for now. It shouldn't change that much however.
It's normal to have very high numbers, the goal is mainly to compare the RAID types or to see if the number isn't too low (like 1 year for example), not to use it as an absolute value ;)