The SDA and Jehovah's Witnesses come from the same branch of Adventism that sprung fourth from the Second Great Awakening in America during the 19th century.
Both groups come from the Adventist teachings of William Miller whose predictions on the Second Coming of Jesus caused the Great Disappointment of October 22, 1844. The Adventists have survived, but the SDA and the Jehovah's Witnesses were two groups that successfully managed to put a spin on the Great Disappoint as to offer an excuse for why Jesus did not return in 1844.
Like JWs, the SDA believes in the Marcionism concept that written Scripture is the ultimate source of divine revelation, and as such that it is the basis for all religious doctrine and truth. They believe that God and Satan along with humanity are players in a drama over God's rightful sovereignty, something the SDA calls "The Great Controversy." While they accept the teaching of the Trinity, they do believe in a Christology that matches very much what Witnesses teach in regards to the passion and death of Christ. They too believe in "soul sleep," which seems to be an earmark of all Adventism. And though SDA emphasizes the observance of the Jewish Sabbath for Christians, they too believe that God exists in the same time frame as humans and that "time prophecies" point to the "fact" that we are in the Last Days.
Recently, like the Latter-Day Saints, the SDA has been redefining its beliefs via critical analysis such as is done in Catholicism and Judaism. This has allowed them to be more honest about their past statements that, like the JWs, have failed to come to pass and seem to lean in the direction of claiming a past over-emphasis on the works of Ellen G. White. Hoping that adopting critical analytical hermeneutics would prove as advantageous as it has over the past centuries for both Catholicism and Judaism, both the LDS and SDA have experienced the reverse and now see a drop in growth.
A footnote to all this is that the Bahá'í Faith, a mixture of religious teachings from East and West, claims that Miller was generally correct about his calculations regarding the Second Coming. The 1844 calculations are believed by the Bahá'í to correctly forecast the arrival of the Báb, a major figure in their religion.
While the Jehovah's Witnesses are not a direct splinter of the Millerite movement, Charles Taze Russell once commented: "I confess indebtedness to Adventists." (wt 6/1/1916, p 170) The Bible Students and subsequently the JWs employed Adventist views on calculating time periods from numbers mentioned in Scriptures, adopting their general ideas that the final days of the world began in the 19th century and that the last generation of humanity was now present since the mid-1800s.