The Millerites were the followers of the teachings of William Miller, who in 1831 first shared publicly his belief that the Second Advent of Jesus Christ would occur in roughly the year 1843–1844. Coming during the Second Great Awakening, his teachings were spread widely and grew in popularity, which led to the event known as the Great Disappointment.
The beginning of the 2,300 days: The decree of Artaxerses in the 7th year of his reign (457 BC) as recorded in Ezra marks the beginning of the 2300 days. King's reigns were counted from New Year to New Year following an Accession Year. The Persian New Year began in Nisan (March–April). The Jewish civil New Year began in Tishri (September–October).
William Miller (preacher)
William Miller was an American clergyman who is credited with beginning the mid-19th-century North American religious movement known as Millerism. After his proclamation of the Second Coming did not occur as expected in the 1840s, new heirs of his message emerged, including the Advent Christians (1860), the Seventh-day Adventists (1863) and other Adventist movements.
William Miller (preacher)
William Miller's Low Hampton, New York home
Beginning of the 70 weeks: The decree of Artaxerxes I of Persia in the 7th year of his reign (457 BC) as recorded in Ezra marks beginning of 70 weeks. Kings' reigns were counted from New Year to New Year following an "Accession Year". The Persian New Year began in Nisan (March–April). The civil New Year in the Kingdom of Judah began in Tishri (September–October).
A chart showing Miller's calculations which mark the Second Coming at 1843.