And the Lord God commanded the man, “You are free to eat from any tree in the garden; 17 but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.” NIV
The NIV does not make this implication. Perhaps you would get better answers on Hermeneutics.
KJV was written in English for the common man ... in the 17th century! English and many of its words have changed much since. It is entirely possible that this has to do with the KJV practically being an ancient manuscript that should be read from the perspective of its intended audience.
Since Adam and Eve did not die on that exact day it is safe to say that the translators did not intend to give that impression in Genesis 2:17. So there must be another explanation of what they meant by it. The most likely, I would say is that the word day at the time or in this context does not necessarily mean between sun up and sun down. What it means exactly in this case I cannot say. That is why I suggest the Hermeneutics site.
Some might argue that die means something other than actual death, but I really don't buy that, but that is strictly my opinion and not really part of your question. If you take one of those perspectives it could easily answer this question for you, though.