Is not the use of the term "cubit" at Matthew 6:27, New World Translation, wrong since a life span cannot be measured by a cubit?

Matthew 6:27 reads: "Who of you by being anxious can add one cubit to his life span?" The Greek word here is pe'khus,which literally means the forearm, on which the cubit was based, and is therefore the Greek word for cubit. Luke used the same word when recording these words of Jesus, and John used it in describing how far the disciples were from shore when they got the great catch of fish when Jesus appeared to them after his resurrection. The same word he also used in giving the measurements of the holy city, Jerusalem.- Luke 12:25; John 21:8; Rev. 21:17.

Obviously, then, the thought of pe'khus is that of length. In making use of this term Jesus, in his sermon on the mount, was not discussing the stature of one's body or its height or tallness, for that is not a common source of anxiety. Rather, he was referring to the prolongation of one's life. Life is measured by its length, as indicated by the use of the phrase " life span" in the New World Translation. Therefore a measure of length, namely, a cubit, which was eighteen inches long, is very appropriate and, compared with the length of life, it would certainly be very short. This was the point that Jesus was making: by being anxious you cannot increase your life span even eighteen inches. But to add eighteen inches to one's height would be phenomenal, making one a giant, as compared to others.

It is very interesting to note that An American Translation, by Smith and Goodspeed, renders Matthew 6:27 as follows: " But which of you with all his worry can add a single hour to his life?" The Revised Standard Version of 1952 renders the verse: "And which of you by being anxious can add one cubit to his span of life?" The Roman Catholic Spencer Version reads: " Yet who among you, by anxious thought, is able to add a single span to his life?" A modern American version by C. Williams freely translates Jesus' words thus: "But which of you by worrying can add a single minute to his life?" A footnote states: "The word means size or time; here time." And the Diaglott, an interlinear Greek version reads:"Besides, which of you, by being anxious, can prolong his life one moment?" Its interlinear, word-for-word translation reads: "Which and by of you being over careful is able to add to the age of him span one?" Clearly, in view of all the foregoing, Matthew 6:27 is properly rendered in the New World Translation. - Watchtower, Question from Readers, 1961, May 1st, p.287