In other words the Book of Psalms as we have it today was the result of a process spanning about 1000 years. It began with individual psalms, the earliest being Psalm 90 by Moses (ca. 1400 B.C.). More than half were written by David (ca. 1000 B.C.). Then the individual psalms were grouped into collections of books for corporate worship, and finally the books were arranged into the final book, probably around 444 B.C. (Ezra’s time).