Allow substraction of years (NOW - x)

Hi @tim-diels
I have checked the #2 patch and it works. But there is a way to break functionality by setting substraction value big enough to make "to year" earlier, than "from" year. Let's add some validation.

Thanks for the work here! The lack of flexibility in defining year ranges will be addressed with different approach that should accommodate this and other issues attempting to do similar things. Please ✨ Allow flexible "from" and "to" (min/max) year ranges Needs work for details.

The hope is to provide a conversion path for any config generated by using this patch. Credit will be granted in the other issue. Please feel free to follow/review/contribute there.