I've been a JavaScript developer since the nineties and I too have been puzzled about this very thing. Everyone has known Date has been broken for a very long time and a plethora of pollyfills and datetime libraries have sprung up to band-aid the situation but nothing ever got close to being resolved as major ECMAscript versions were released over the years. I guess if it takes 270 pages for MDN to explain it, it's a rocket science problem that's well over my head.