There are multiple contrasts in the proverb: one fears God and therefore keeps his commandments, the other has no fear and lives carelessly; one has something prolonged, the other has something shortened; in one case it is days that are added to , and in the other it is years that are reduced. The last of these contrasts is striking because it seems to make the comparison unequal, but this is the key.
Here is a contrast between the life which is lived in awe of God, in reverence for Him, seeking to obey Him, and the life which is not. It does not matter how long or short the life is, the days are either highly significant, or they are cut down. So, let's always value awe of God and subordination to him and fear of him and obedience; let us practice great watchfulness of our lives and fear of offending him, and it will result in every day being significant. Every day we shall get new wisdom. Every day we shall get some burst of light and understanding on the word of God. Every day we shall have gratitude and worship and fruitfulness. But with those who lose all awe of God, and this could apply to Christians also: somebody who loves the Lord, yes, but you lose your fear and your awe and you begin to sin. You live as though he cannot see, as though he will not judge, as though he is not offended, as though it does not matter. You begin to be slip-shod in your Christian life and you no longer practise self-examination and regular repentance of sin. The months and the years can pass and they will be docked off, made small, as though a great scythe came and cut through the plants, and your advance in holiness and understanding will be stunted.
One or two of the modem translations go in for a little bit of interpretation here, and say ‘The fear of the Lord prolongeth life’, which is a great shame, because they ruin the proverb in so doing. No, this is days and years, and there lies the key to understanding this particular parable. Because you may think that taken literally the promise would be that awe and reverence for God has this effect that you should live a long life, whereas wickedness will mean that you will live a short life. We might also think of godly Stephen, the first martyr of the New Testament, and how his life was cut short. The promise is not for a long life in every case, but for a profitable life.