Not sure if the Jewish Encyclopedia is a good enough source, but it does have some good insight on Numbers in the Bible. Numbers had a very symbolic meaning to the ancient Israelites and certain concepts became associated with certain numbers. Therefore they tended to round or try to find ways to describe things in terms of an appropriate number based on the symbolism of what they were trying to express.
For example, 4 represented completeness (4 cardinal points on the compass 4 letters in the Hebrew writing of Jahweh, etc). So, for example, in Ezekial 14:12 we read ""For this is what the Sovereign LORD says: How much worse will it be when I send against Jerusalem my four dreadful judgments--sword and famine and wild beasts and plague--to kill its men and their animals!" God's 4 judgments it will be a complete and total devastation. 40 typically meant a complete generation or a complete change so 40 years in the desert meant that the Israelites wandered for a complete generation. Sometimes they would use multiples of sacred numbers. So we see in the genealogies of Jesus that the author plays fast and loose with the tradition to make sure that he can say: "So all the generations from Abraham to David are fourteen generations; from David to the deportation to Babylon, fourteen generations; and from the deportation to Babylon to the Messiah, fourteen generations." (Matthew 1:17). 14 is 7 times 2 which means he can still invoke the sacredness of the number 7 which is associated with the creation, the Sabbath, the Temple and many other things thereby "proving" that Jesus is important because he fits into the Biblical pattern.
Just for the record "40" does not necessarily mean 40 of any given time (days or years) but instead just a "very long period of time" of testing or judgment. For instance the rain falling for forty days and forty nights during the flood, the forty days that Moses spent in the wilderness, the forty years the Israelites spend in the wilderness and the forty days that Jesus spends in the desert. It does not mean literally forty days or years, just an indeterminately long time.