The last time Moroni gave us a date was in Mormon 8:6, where it was four hundred years since the coming of Christ. At that time, he had said “I have not friends or whither to do; and how long the Lord will suffer that I may live I know not” (Mormon 8:5).
Now, twenty years later, Moroni is sure of the fact that it is time to “seal up these records.” Only at the very end of the record do we see that perhaps Moroni is nearing the end of his days (see verse 34 in this chapter).
After twenty years of being surprised that he was still alive and adding more text to his father’s book, the time has come to close it. While it is still his father’s book, he has cared for it for longer than it was in Mormon’s care (based on reconstruction of events). Mormon ended his work with a plea to future Lamanites (Mormon 7), and Moroni fittingly does the same.