Moffatt puts it "before the Son of man arrives" as if Jesus referred to this special tour of Galilee. Jesus could overtake them. Possibly so, but it is by no means clear. Some refer it to the Transfiguration, others to the coming of the Holy Spirit at Pentecost, others to the Second Coming. Some hold that Matthew has put the saying in the wrong context. Others bluntly say that Jesus was mistaken, a very serious charge to make in his instructions to these preachers. The use of ἑως with aorist subjunctive for a future event is a good Greek idiom.