Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
Introduction to Numbers
The title commonly given to this book is evidently suggested by the two numberings of the people recorded in Num. 1 and Num. 26.
The book narrates the history of the Israelites during their sojourn in the wilderness from the completion of the law-giving at Sinai, Lev 27:34, to their mustering in the plains of Moab for actual entry into the land of promise.
The incidents are generally given in their chronological order, except in the third part. The five chapters comprised in this part appear to deal with a long period, from which only isolated episodes are given; and of these the dates can only be conjectured.
Between the two dates "the first day of the second month of the second year after they were come out of Egypt" Num 1:1, and the death of Aaron Num 33:38, intervene no less than 38 years and 3 months (compare Deu 2:14), the long and dreary period of tarrying in the wilderness until the disobedient generation had wasted away.
From the death of Aaron to the date given in the opening verses of Deuteronomy Deu 1:1-3, occurred a space of exactly six months, in which all the events narrated in the fourth part of the Book of Numbers, from Num 20:1 to the end, would seem to have occurred, with the probable exception of the defeat of the king of Arad Num 21:1-3.
As regards the authorship and date of composition, the notes of time, the tenor of the contents, no less than the direct assertions of the text itself, lead to the conclusion that Moses is properly spoken of as the writer of the Book of Numbers. It is in substance his work; though many portions of it were probably committed to writing many years before the whole was completed; and the concluding chapters were not written until toward the close of the 40th year after the exodus.