In 1870, Charles Taze Russell and others formed a group in Pittsburgh, Pennsylvania, to study the Bible. During the course of his ministry, Russell disputed many beliefs of mainstream Christianity including immortality of the soul, hellfire, predestination, the fleshly return of Jesus Christ, the Trinity, and the burning up of the world. In 1876, Russell met Nelson H. Barbour; later that year they jointly produced the book Three Worlds, which combined restitutionist views with end time prophecy. The book taught that God's dealings with humanity were divided dispensationally, each ending with a "harvest," that Christ had returned as an invisible spirit being in 1874 inaugurating the "harvest of the Gospel age," and that 1914 would mark the end of a 2520-year period called "the Gentile Times," at which time world society would be replaced by the full establishment of God's kingdom on earth. Beginning in 1878 Russell and Barbour jointly edited a religious journal, Herald of the Morning. In June 1879 the two split over doctrinal differences, and in July, Russell began publishing the magazine Zion's Watch Tower and Herald of Christ's Presence, stating that its purpose was to demonstrate that the world was in "the last days," and that a new age of earthly and human restitution under the reign of Christ was imminent.

When did Charles Taze Russell form a group?