According to the Encyclopedia of Mormonism, chapter summaries first appeared in an edition published in 1920, under the editorship of James E. Talmage, of the Quorum of the Twelve Apostles, but the chapter summaries in modern printings of the Book of Mormon, as well as the footnotes and cross-references, date to the 1981 printing, which was "edited by a committee headed by members of the Quorum of the Twelve."
These chapter summaries were not part of the original text, and are intended simply as summaries, much like those that can be found in many editions of the Bible, giving an overview of the material to be discussed.
As for the dates, they simply count years in the narrative from fixed reference points in time, under the assumption that the birth of Jesus happened in the year 1 AD. (This date is used for convenience, and is not an actual point of doctrine, so far as I know.) The beginning of the book is set at 600 BC, because of a revelation that the Savior would be born six hundred years after their departure from Jerusalem. The author(s) of the dates clearly did not want to embellish in any way; in spans where the text does not count years, the date on that page simply says "between X and Y," giving the two known years from the chronology that the current events of the narrative are taking place somewhere between.