Moses prays to the eternal God whose years have no end, contrasting Him with man, whose life is short and quickly swept away like grass that flourishes in the morning and withers by evening. Because life is brief and often spent under toil, Moses asks God to teach His people to number their days rightly, so wisdom, not carelessness, shapes how they live.
The sermon carries this into everyday assurance: the same eternal God has also promised never to forget His people, engraving them on the palms of His hands and causing all things to work together for the good of those who love Him. Numbering our days well means trusting that unshakable care while our days last.