We understand that our days of living are known to the Lord and that our time of death are also already known by Him. Can we die before our appointed time?
Let me preface my answer with this. I’m assuming you were referencing D&C 122:9 which says:
Therefore, hold on thy way, and the priesthood shall remain with thee; for their bounds are set, they cannot pass. Thy days are known, and thy years shall not be numbered less; therefore, fear not what man can do, for God shall be with you forever and ever.
I don’t think it’s so clear-cut whether this scripture is a personal promise given to Joseph Smith at a terribly low time in his life, or if it’s generally applicable.
The most important doctrine in the gospel of Jesus Christ is our “moral agency.” Our moral agency allows us to make choices according to the knowledge we have. The time of death that is known to our Father in heaven is more fluid than static.
If I am living according to the commandments of God the time of my death is appointed, and yet, it appears from scripture that an appointed time of death can even be extended should we exercise enough faith. In our “Guide to the Scriptures” as provided on Lds.org we have the following statement regarding a righteous king Hezekiah,
Hezekiah’s life was extended fifteen years through prayer and faith. (2 Kgs. 20:1–7)
One of the tragedies of this though is that in that extended period of time King Hezekiah bore a son who ended up killing Isaiah, which then leads to the question what would have happened to Isaiah if the King would have accepted his death when it was appointed?
Also, in light of our moral agency, the time of our appointed death could be lessened if we should make decisions that would cause an untimely death. For example, if I were to decide to jump off a cliff, even if I was living righteously, my time of death may come a lot sooner than was appointed. Our Father in heaven may have known such would happen, and that my decision would be such, but I am under the assumption that our true appointed time of death is according to using our agency “wisely” also. Let me further clarify, if I have been taught that drugs may kill me if I partake of them, and despite this knowledge I partake of the drugs. I have now provided an opportunity where I might cause my body an experience that will take me before my appointed time if I had been living according to the commandments given by God.
Let’s look at one of Satan’s temptations to Jesus Christ pertaining to his decision, his choice, and scripture. We all know our Savior had an appointed time of death — His atonement, and with knowledge of this here is Satan’s temptation,
Then the devil taketh him up into the holy city, and setteth him on a pinnacle of the temple,
And saith unto him, If thou be the Son of God, cast thyself down: for it is written, He shall give his angels charge concerning thee: and in their hands they shall bear thee up, lest at any time thou dash thy foot against a stone.
Jesus said unto him, It is written again, Thou shalt not tempt the Lord thy God. (Matthew 4:5-7)
I think it wise that we do not tempt God with anything especially our appointed time of death by making a decision that we know may end our life sooner than the appointed time.
In my honest opinion, people who are taken from this earth, to no fault of their own (i.e. car accidents) and who have been living righteously, have passed away according to God’s will and pleasure — their appointed time. Why? I don’t know, but I trust God knows, and I trust in his wisdom.