At a time when many people still believed that the earth was created ex nihilo (out of nothing) within the last 10,000 years - and some still believe this - Joseph Smith stated:"You ask the learned doctors why they say the world was made out of nothing; and they will answer, "Doesn't the Bible say He created the world?" And they infer, from the word create, that it must have been made out of nothing. Now, the word create came from the word baurau [Hebrew] which does not mean to create out of nothing; it means to organize; the same as a man would organize materials and build a ship.
"Hence, we infer that God had materials to organize the world out of chaos... The pure principles of element are principles which can never be destroyed; they may be organized and re-organized, but not destroyed." - Joseph Smith, Jr., April 6, 1844.This may be compared with a statement on conservation of mass-energy from a Physics textbook: "[Mass-] energy may be transformed from one kind to another, but it cannot be created or destroyed." Fundamentals of Physics, Halliday and Resnick, 1981.
W.W. Phelps quoted Joseph Smith, Jr., in the Times and Seasons as follows:"Well, now, Brother William, when the house of Israel begin to come into the glorious mysteries of the kingdom, and find... that eternity, agreeably to the records found in the catacombs of Egypt, has been going on in this system, almost two thousand five hundred and fifty five millions of years [that is, a few billion years]: and to know at the same time, that... geologists and others are trying to prove that matter must have existed hundreds of thousands of years;.. see and know as we are seen and known!" (Times and Seasons, Nauvoo, vol. 5, pg. 758 (Dec. 25, 1844)).
Another comment on the ancient age of the earth's system is found in a speech given by President David O. McKay at BYU on 30 October 1956"And now I have just time to comment on the opportunity of the BYU to teach these fundamental truths. This thought was expressed by Dr. Sidney B. Sperry in the opening prayer, that here in this school, destined to become the greatest in the world, opportunities are given to guide students in this higher quality of life, this guide, this anchor, this cord leading into the depths of the forest. Whatever the subject may be, the principles of the gospel of Jesus Christ may be elaborated upon without fear of anyone's objecting, and the teacher can be free to express his honest conviction regarding it, whether that subject be in geology, the history of the world, the millions of years that it took to prepare the physical world, whether it be in engineering, literature, art..."
Finally, from the Discourses of Brigham Young, pages 258-259:"I am not astonished that infidelity prevails to a great extent among the inhabitants of the earth, for the religious teachers of the people advance many ideas and notions for truth which are in opposition to and contradict fact demonstrated by science, and which are generally understood. You take, for instance, our geologists, and they tell us that this earth has been in existence for thousands and millions of years. They think, and they have good reason for their faith, that their researches and investigations enable them to demonstrate that this earth has been has been in existence as long as they assert it has; and they say, "If the Lord, as religionists declare, made the earth out of nothing in six days, six thousand years ago, our studies are all vain; but by what we can learn from nature and the immutable laws of the Creator as revealed therein, we know that your theories are incorrect and consequently we must reject your religions as false and vain; we must be what you call infidels, with the demonstrated truth of science in our possession; or, rejecting those truths, become enthusiasts in, what you call, Christianity."
"In these respects we differ from the Christian world, for our religion will not clash with or contradict the facts of science in any particular. You may take geology, for instance, and it is true science; not that I would say for a moment that all the conclusions and deductions of its professors are true, but its leading principles are; they are facts--they are eternal; and to assert that the Lord made this earth out of nothing is preposterous and impossible. God never made something out of nothing; it is not in the economy or law by which the worlds were, are, or will exist. There is an eternity before us, and it is full of matter; and if we but understand enough of the Lord and his ways we would say that he took of this matter and organized this earth from it."