Psalms 90:15

Make us glad according to the days wherein thou hast afflicted us, and the years wherein we have seen evil.

It is a prayer asking God to answer past affliction and years of seeing evil with a corresponding measure of gladness. The gladness requested is proportioned to the days of being afflicted and to the years of experiencing evil. It is sought together with God's mercy, so that His servants and His people may rejoice through all their days and throughout their lives.